Abstract:
A method for controlling an electron beam process wherein a wire is melted and deposited on a substrate as a molten pool comprises generating the electron beam with a complex raster pattern, and directing the beam onto an outer surface of the wire to thereby control a location of the wire with respect to the molten pool. Directing the beam selectively heats the outer surface of the wire and maintains the position of the wire with respect to the molten pool. An apparatus for controlling an electron beam process includes a beam gun adapted for generating the electron beam, and a controller adapted for providing the electron beam with a complex raster pattern and for directing the electron beam onto an outer surface of the wire to control a location of the wire with respect to the molten pool.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application 61/173,292, filed on Apr. 28, 2009, which is hereby incorporated by reference in its entirety. In addition, this application is co-pending with the related application entitled “CLOSED-LOOP PROCESS CONTROL FOR ELECTRON BEAM FREEFORM FABRICATION AND DEPOSITION PROCESSES,” U.S. application Ser. No. 12/750,991, filed on the same day and owned by the same assignee as this application, the contents of which are incorporated herein by reference in their entirety. 
     
    
     ORIGIN OF THE INVENTION 
       [0002]    This invention was made in part by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates to a method and apparatus for controlling a beam position during an electron beam wire deposition process. 
       BACKGROUND OF THE INVENTION 
       [0004]    Electron beam freeform fabrication or EBF 3  is an emerging manufacturing deposition process in which an electron beam is used in conjunction with a wire feed in order to progressively build material on a substrate in a layered manner. The electron beam is translated with respect to a surface of the substrate while the wire is melted and fed into a molten pool. In an EBF 3  process, a design drawing of a three-dimensional (3D) object may be sliced into different layers as a preparatory step, with the electron beam tracing each of the various layers within a relatively high-vacuum environment. The layers cool into a desired complex or 3D shape. 
         [0005]    Conventional electron beam control methodologies may be less than optimal for certain purposes, such as for maintaining an even or consistent material deposition height. In addition, manual controls are often used to retain the wire feedstock as it is fed into and captured in the beam and the molten pool. Perturbations may cause the wire to stray from the beam path and/or the molten pool, potentially causing transient instability and discontinuities in the deposited material. Moreover, convention deposition control processes perform a single process at a time, modulating the electron beam between processing steps for serial application of different techniques. With the development of EBF 3 , control processing complexity has increased dramatically. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, a control method and apparatus are set forth herein for an electron beam process, e.g., electron beam welding and electron beam freeform fabrication (EBF 3 ). The method, which is executable via the control apparatus set forth herein, provides for complex rastering of an electron beam generated in a vacuum, such as the chamber of an electron beam gun, and enables several processing functions or tasks to be performed simultaneously in parallel rather than in series. Thermal input into the EBF 3  process results in self-correcting control and steering of a wire with respect to a molten pool formed during the process. 
         [0007]    The present invention contemplates a splitting of the duty cycle of the electron beam during rastering. This in turn enables real-time control of electron beam processing, along with the simultaneous achievement of multiple tasks or objectives. Process control is thus optimized at lower relative power consumption levels while minimizing undesirable process issues, e.g., selective vaporization of low vapor pressure alloying elements, thermal residual stress, and distortion, associated with excessive thermal input into components during electron beam processing. Beam rastering as set forth herein may dramatically reduce the number of flaws encountered, particularly during EBF 3 . The method uses beam deflection to preheat and steer the wire into the molten pool, thus reducing a primary flaw source when the wire exits the pool. 
         [0008]    The self-corrective method provides for control over the location, power, and dwell time of the electron beam to generate a complex raster pattern, and uses the raster pattern to control thermal input and distribution. The raster pattern redirects the wire, and retains a position of the wire with respect to the molten pool by focusing the beam on the outside of the wire. This selectively heats the outer edges of the wire, and prevents straying of the wire from the molten pool. The method is “self-correcting” in that it maintains the wire in the pool without requiring sensing or external changes to the raster pattern or wire orientation. 
         [0009]    In particular, a method is provided for controlling an electron beam process, e.g., welding and EBF 3 , wherein a wire is melted by the heat of an electron beam and deposited as a molten pool on a substrate, where the molten pool cools to form a layer. The method includes generating the electron beam with a complex raster pattern or patterns, and directing the electron beam with its complex raster pattern onto outer surfaces of the wire to thereby steer the wire with respect to the molten pool. Directing the electron beam locally preheats the wire, thereby retaining a position of the wire, again with respect to the molten pool. 
         [0010]    An apparatus for controlling an electron beam process is also provided, with the apparatus including an electron beam gun, adapted for generating the electron beam, and a controller. The controller is in communication with the electron beam gun to provide the electron beam with the complex raster pattern, and to direct the beam onto an outer surface of the wire. In this manner, a location or position of the wire is controlled and maintained with respect to the molten pool. 
         [0011]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic illustration of elements of an electron beam freeform fabrication (EBF 3 ) apparatus in accordance with the invention; 
           [0013]      FIG. 2  is a schematic illustration of one possible embodiment of a raster pattern usable with the apparatus of  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic illustration of an electron beam raster pattern, such as of the type shown in  FIG. 2 , being used as a wire guide during an EBF 3  process; and 
           [0015]      FIG. 4  is a flow chart describing the method of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring to the drawings wherein like reference numbers represent like components throughout the several figures, and beginning with  FIG. 1 , an apparatus  10  is configured for forming a product using an electron beam  14 . Such a process may include beam welding, or, in another embodiment, may include electron beam freeform fabrication, hereinafter abbreviated as EBF 3  for simplicity. The apparatus  10  includes an electron beam gun  12  adapted to generate the electron beam  14 . While the electron beam  14  is shown external to the electron beam  12  for clarity, those of ordinary skill in the art will recognize a vacuum chamber (not shown) is present within which the electron beam  14  is ultimately generated and contained. 
         [0017]    The apparatus  10  includes a wire feeder  16  adapted for feeding a length of consumable wire  18  toward a substrate  20 , and a controller (C)  22 . The substrate  20  may be positioned on a moveable platform (not shown), with the platform and/or the gun  12  being movable via a multi-axis positioning system (not shown). Alternately, the electron beam gun  12  may be completely enclosed within the vacuum chamber so that the electron beam gun is also moved rather than just the substrate  20 . In either embodiment, relative motion occurs between the electron beam gun  12  is and the substrate  20 . 
         [0018]    The wire  18  is typically a suitable metal such as aluminum or titanium, although the actual material may vary depending on the desired application. Controller  22  includes an algorithm  100  adapted for controlling the EBF 3  process conducted by the apparatus  10 . Controller  22  is electrically connected to the electron beam gun  12 , and adapted to transmit control signals  11  thereto for control of certain operations of the gun and the electron beam  14 , which ultimately melts the wire  18  into a molten pool  24  and deposits it on a substrate  20 , where it ultimately cools to form a layer of a product. 
         [0019]    The wire  18 , when sufficiently heated by the electron beam  14 , e.g., to over approximately 3000° F. in one embodiment, is accurately deposited, layer upon layer, using a set of design data  19 , e.g., Computer Aided Design (CAD) data or another 3D design file. In this manner, a 3D structural part may be created in an additive manner without the need for a casting die or mold. Rapid prototyping and hands-free manufacturing of vehicle, airplane, spacecraft, and/or other complex components or parts is thus enabled. 
         [0020]    Still referring to  FIG. 1 , the controller  22 , using the algorithm  100  described below with reference to  FIG. 4 , is adapted to control the electron beam gun  12  via automatic modulation of the electron beam  14  or otherwise in order to generate a sufficiently complex raster pattern. As used herein, the term “complex raster pattern” refers to a pre-programmed pattern having multiple shapes together, e.g., as shown in  FIG. 2 , or an adaptive control loop wherein the shape of the raster pattern is changed. In the latter pattern, the raster pattern may be linked to the geometry of a CAD or other design drawing being built, or the raster pattern is otherwise actively modified by controller  22 . 
         [0021]    One possible embodiment of a usable raster pattern is shown as pattern  30  in  FIG. 2  and described below. However, the exact raster pattern may vary widely without departing from the intended inventive scope. Whatever shape the raster pattern ultimately takes, the pattern partitions beam energy of the electron beam  14  before entering the molten pool  24 , thus preheating the substrate  20  in advance of the molten pool and preheating the wire  18  used as feedstock during the deposition process. 
         [0022]    During EBF 3  processing, the beam rastering capability of controller  22  is orders of magnitude faster than the thermodynamic rate of the wire melting process. so part of its duty cycle may be used to redirect the electron beam  14 , e.g., for fractions of a second, without impairing the deposition process. This diversion may allow the electron beam  14  to be focused at different locations to simultaneously achieve a variety of effects. The electron beam  14  of  FIG. 1  can also be split into multiple beams for parallel processing. For example, in addition to performing deposition as shown in  FIG. 1 , the beam  14  may also be diverted to: (1) preheat the substrate  20  or to deposit material in front of the molten pool  24 , which may enhance fusion between the deposited material and the substrate, and which may help to control deposit geometry as shapes change; (2) preheat the wire  18  before it enters the molten pool  24 , thus reducing the total amount of energy required to melt the wire  18 , thereby reducing total power input into the EBF 3  process, thereby reducing overheating, induced residual stresses, and loss of low vapor pressure alloying elements; (3) control the position of the wire  18  through differential heating; (4) control or change the shape and temperature distribution of the molten pool  24 ; (5) heat treat the immediate and distant vicinity of the molten pool  24  to eliminate thermal residual stresses and distortion; and (6) in-situ process observation of the electron beam  14 , and detection of diffracted or secondary electrons using an electron beam detector, such as those used on scanning electron microscopes. 
         [0023]    Referring to  FIG. 2 , the present invention provides unique beam raster patterns and successfully demonstrates techniques (1-6) noted above. An example of one such raster pattern is shown as pattern  30 . Pattern  30  may include different patterns of varying size and/or complexity, e.g., a round pattern portion  32  and a triangular pattern pattern  34 . The round pattern portion  32  of pattern  30  is intended to keep a substantially circular-shaped molten pool  24 , i.e., item (4) in the list cited above. The triangular pattern portion  34  may be focused on preheating the wire  18  immediately in front of the molten pool  24 , and on preheating the wire during the deposition process. See items (1) and (2), respectively, in the list cited above. Pattern  30  of  FIG. 2  uses dwell time to control the partitioning of the incident energy of beam  14  in the molten pool  24 , on the wire  18 , and in advance of the molten pool  24  to minimize the thermal input into the EBF 3  process, while still maintaining high quality deposits on the substrate  20 . 
         [0024]    Referring to  FIG. 2 , advanced beam rastering can be used to direct the energy into the wire (item 2 above), and maintain process continuity through self correction (item 3 above). This may be achieved by controlling the location, power, and dwell time of the beam  14  to precisely control the thermal input and distribution in the apparatus  10  of  FIG. 1 . This is a direct result of the particular raster pattern used to deflect the electron beam  14 . 
         [0025]    Referring to  FIG. 3 , some beam rastering patterns may be able to redirect the wire  18  to keep it in the molten pool  24  (see  FIG. 1 ). This may be achieved through a focusing of the electron beam  14  on the outsides of the wire, as indicated by the intense scan region  21 . Selective heating of the outer edges of wire  18  is provided when the wire would otherwise stray from the molten pool  24  of  FIG. 1 . For example, in  FIG. 3 , if the wire  18  is straying too far to the right, modifying the beam raster pattern to increase the scanning intensity and dwell time over the intense scan region  21  will have the effect of selectively heating the outer edge of wire  18  to push the wire back to the left into the molten pool  24 . This approach requires monitoring of the wire  18  position relative to the molten pool  24  and a modification to the raster pattern to correct the wire  18  position as necessary to maintain a consistent process. 
         [0026]    As a result of a change in stiffness due to localized heating, the wire  18  automatically curls away from heat applied to its outer edge and back into the molten pool  24 . The net result is that with a fixed beam raster pattern where an intense scan region  21  is continuously maintained on both right and left sides of the wire  18 , i.e., one that is not being externally changed to react to the wire position, the process becomes self-correcting. This maintains the position of the wire  18  with respect to the molten pool  24  of  FIG. 1  without requiring sensing or external changes to the raster pattern, e.g., pattern  30  of  FIG. 2 , or wire orientation to maintain process consistency. 
         [0027]    Referring to  FIG. 4 , and with reference to the structure of the apparatus  10  shown in  FIG. 1 , algorithm  100  begins at step  102 , wherein the controller  22  controls the electron beam gun  12  to generate the complex raster pattern, e.g., pattern  30  shown in  FIG. 2 . For example, the controller  22  may automatically modulate the electron beam  14  or use other suitable means in order to generate a desired complex raster pattern as described above. The algorithm  100  then proceeds to step  104 . 
         [0028]    At step  104 , the controller  22  diverts the electron beam  14  for a portion of the duty cycle. As noted above, the beam rastering capability of controller  22  is orders of magnitude faster than the thermodynamic rate of the wire melting process, so beam  14  may be redirected for fractions of a second without impairing the deposition process. The algorithm  100  then finishes with step  106 , wherein the required EBF 3  tasks are executed using the diverted portions of the beam  14 . 
         [0029]    Step  106  may entail various different sub-steps  106 A- 106 D. For example, step  106 A allows for the pre-heating of wire  18  and/or the substrate  20  in advance of the molten pool  24  using diverted beam energy. Step  106 B may include positioning or guiding of the wire  18  as shown in  FIG. 3  and as explained above. Step  106 C can include controlling desired qualities of the molten pool  24 , e.g., shape and/or position. Step  106 D can include any other additional EBF 3  tasks at hand, such as but not limited to heat treating the immediate and distant vicinity of the molten pool  24  to eliminate thermal residual stresses and distortion as noted above, in-situ process observation of beam  14 , etc. 
         [0030]    The innovations in this disclosure are at least twofold. First is the concept of splitting the duty cycle of the electron beam via beam rastering. Second, due to the first concept, a beam rastering technique is provided that dramatically reduces the number of flaws encountered during EBF 3 . This concept uses beam deflection to preheat and steer the wire  18  into the molten pool  24 , reducing one of the primary sources of flaws from the wire pushing out of the molten pool. The innovation in this disclosure can be directly applied to EBF 3  systems, such as the apparatus of U.S. Pat. No. 7,168,935, to improve the control of the wire and the molten pool during EBF 3  processing. 
         [0031]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.