Abstract:
Apparatus ( 20 A-C) and a method for determining and correcting a deformation in an article ( 44 ). An energy beam ( 29 ) such as a laser beam is directed to an area ( 42 A-C) to reverse ( 46, 72, 74 ) an existing deformation or to control deformation during additive fabrication ( 86, 88 ). Two sectionally curved areas of a deformation ( 50 A/ 50 C,  52/54 ) may be heated simultaneously to flatten a bulge between them. An existing or developing deformation may be determined by surface scanning ( 40 ) and/or a deformation may be determined predictively to pro-actively correct and prevent it while building or rebuilding a portion of the article by additive fabrication.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to apparatus and processes for correcting deformations in metal components by selective heating with an energy beam such as a laser beam, and particularly to correction of deformations in gas turbine components. 
       BACKGROUND OF THE INVENTION 
       [0002]    Manufacturing or repair of parts often requires heating the parts. This can result in strain and distortion of the part. For example, welded fabrications are subject to distortion resulting from shrinkage strains during weld metal solidification. In some alloys, micro-structural transformations in the heat affected zone strain the material and contribute to distortion. Other distortions result from service. Residual fabrication stresses can be relieved by elevated temperature operation, resulting in geometric changes in the part. Also, creep can occur from steady state or cyclic stresses experienced by parts over time at elevated temperatures. Manufacturing distortions can be reduced by methods such as strong fixturing, low heat welding, back stepping of weld progression, and chill blocks to minimize heat input to the substrate. Distortion can be partly corrected by plastically bending the component by force. However such restoration is imprecise, can strain harden (cold work) the part, can introduce additional stresses, and can damage the part, especially if it is in a weakened or crack prone condition. 
         [0003]    Heat straightening is another method to correct distortion. A weld between two straight lengths of pipe may result in a bend at the weld. Re-melting the weld on the obtuse side of the bend can introduce weld shrinkage to promote straightening. This is used to straighten fuel injection rockets in combustion support housings of gas turbine engines during original manufacture and during repair operations. Such heat straightening is commonly accomplished using the same weld process (e.g. gas tungsten arc welding) used to make the original weld. Unfortunately, such heat straightening is imprecise. Too much heat over-corrects and too little heat under-corrects the distortion. Welds in sheet metal or large plate fabrications can cause complex and difficult to predict distortions such as buckling or bulging in three dimensions. These are difficult to correct accurately by any known process. 
         [0004]    Lasers offer a source of heat for metal forming and straightening. Some known mechanisms of laser bending of sheet metal include: a) Temperature Gradient Mechanism; b) Buckling; and c) Shortening. These mechanisms are known in the art and are publicly available, so they are not detailed herein. For example, see Section 2.0 of “Laser Assisted Forming for Ship Building” by G. Dearden and S. P. Edwardson, of the University of Liverpool, presented at the Shipyard Applications for Industrial Lasers Forum (SAIL), Williamsburg, Va., Jun. 2-4, 2003. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is explained in the following description in view of the drawings that show: 
           [0006]      FIG. 1  is a schematic view of an apparatus performing a method of the invention. 
           [0007]      FIG. 2  is a top view of a workpiece with a laser heating zone defined by the periphery of a bulge to be flattened, showing two types of laser scan patterns. 
           [0008]      FIG. 3  is a top view of a workpiece illustrating two more types of scan patterns. 
           [0009]      FIG. 4  illustrates a concentric type of laser scan pattern. 
           [0010]      FIG. 5  is a schematic view of a second embodiment of an apparatus performing a method of the invention. 
           [0011]      FIG. 6  is a schematic view of a third embodiment of an apparatus performing a method of the invention on a gas turbine blade as viewed along line  6 - 6  of  FIG. 7   
           [0012]      FIG. 7  is a top view of a gas turbine blade with a dashed outline indicating distortion that would occur from thermal expansion of the pressure side without heat compensation on the suction side during additive processing as shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The present inventors recognized that laser energy can be accurately scanned over one or more defined areas of a metal surface by rastering the beam with mirrors, for precision bending of an article to correct complex distortions thereof. 
         [0014]      FIG. 1  shows an apparatus  20 A performing a method of the invention. The apparatus includes a fixturing mechanism or work table  22 , a controller  24 , a surface imaging scanner  26 , a controllable emitter  28  of an energy beam  29 , and optionally, one or more additional controllable beam emitters  30 . Control lines  32  are indicated by arrows directed toward peripherals L, G from the controller. “L” represents a laser emitter, and “G” represents a galvanometer actuated mirror. Alternately, other types of energy beams and actuators may be used. A sense line  34  is indicated by an arrow directed toward the controller from an image sensor  36 . The imaging scanner  26  may comprise a triangulation laser scanner with a laser emitter  38  that produces a beam  40  scanned across a surface  42  of the workpiece  44  by an actuator such as a galvanometer G, and a camera comprising a lens  45  and a sensor  36  such as a charge coupled device (CCD). Such scanners can currently image a surface in 3 dimensions to a precision of at least 10s of microns or thousandths of an inch. The surface  42  has a central bulge with peripheral areas  42 A,  42 B that are curved in a first direction and a middle area  42 C curved in an opposite direction. 
         [0015]    The controller  24  may be a computer that stores a specification of the surface  42  provided by computer aided engineering software and digital storage media. The workpiece is fixed to the worktable  22  or other fixturing device. The scanner  26  images the surface and provides surface coordinates to the controller. The controller compares the actual surface shape to the specified shape, and determines corrections to be made. In this example, the workpiece has a bulge to be reversed to provide a planar workpiece. This can be done by heating a periphery of the bulge. Parameters of the heating laser(s)  29  determine the direction and degree of corrective bending. In  FIG. 1 , a temperature gradient mechanism is being employed to bend  46  the periphery of the bulge in a direction toward the laser to straighten the workpiece  44  by plasticizing or melting the near side while thermally expanding the far side of the workpiece. 
         [0016]    When removing a bulge as in  FIG. 1  it is beneficial to bend opposite sides of the bulge simultaneously to prevent resistance to the correction on one side by the distorted opposite side. To this end, two laser emitters  28 ,  30  may process respective opposite sides of the bulge periphery simultaneously. Alternately, time sharing of a single source emitter could be performed sufficiently rapidly to heat separate areas on the workpiece. 
         [0017]      FIG. 2  shows a top view of a workpiece  44  with a laser heating zone  48  that has been identified by the controller  24  around the periphery of a bulge to flatten the bulge. This heating zone follows one or more sectionally curved surface areas  42 A,  42 B as seen in a sectional view as in  FIG. 1 . A first type of raster scan pattern ( 50 A-C) forms tracks that are transverse to the bulge periphery. Heating portions  50 A,  50 C are on opposite sides of the laser heating zone  48 . A spanning portion  50 B of the tracks traverse the bulge with the laser turned off or with the laser beam speed of such large magnitude so as to deposit minimal energy over  50 B. Alternately, the spanning portion  50 B may apply a different laser power to the central portion of the bulge to soften it and/or bend it in the opposite direction from the periphery as later described. With this pattern heating can be performed on opposite sides of the bulge periphery effectively simultaneously with a single laser. Herein “effectively simultaneous heating” means heating that progresses concurrently in two separate areas  50 A,  50 C by accumulating heat therein over multiple passes, although the energy beam may not be in both areas at once. A second type of laser scan pattern  52 ,  54  is shown with separate scan patterns on opposite sides of the bulge periphery. These two patterns  52 ,  54  may be applied simultaneously with two lasers as shown in  FIG. 1 . 
         [0018]      FIG. 3  is a top view of a workpiece  44  with a laser heating zone  48  that has been identified by the controller  24  around a periphery of a bulge to flatten a bulge. It shows a third type of laser scan pattern  56  with concentric heating tracks. A fourth type of laser scan pattern  60  has tracks parallel to the periphery of the heating zone  48 . These scan patterns  56  and  60  are may be applied on opposite sides of the bulge periphery either effectively simultaneously or simultaneously using 1 or 2 lasers respectively. Pattern  60  may optionally be scanned continuously around the whole heating zone  48  using one or more lasers. Since laser beams maintain their intensity with distance from the emitter, the emitters can be located at an optimum distance from the workpiece for wide angle coverage thereof. The distance may be sufficient to enable the laser(s) to scan much or all of the heating zone  48  from one emitter position using 2-axis pivoting actuators as in  FIG. 5 .  FIG. 4  shows a laser scan pattern in which the beam  29  follows a first set of concentric tracks  56 A-C about a first center C 1 , then follows a second set of concentric tracks  58 -C about a second center C 2 , and may continue to follow additional sets of concentric tracks about successive centers C 3 -C 6 . Each set of concentric tracks may contain at least 2 concentric tracks, or at least 3, and overlaps with an adjacent set or sets of concentric tracks. For example, the overlap may be about ⅓ of the diameter of the largest track of each set. This pattern provides controllable multi-pass dwell time in a limited area without hot spots on the surface, enabling implementation of a desired heating specification. 
         [0019]      FIG. 5  shows a second embodiment of an apparatus  20 B performing a method of the invention. The apparatus has a fixturing mechanism or work table  22 , a controller  24 , first and second 2-dimensional scanning laser emitters  62 ,  64 , and a surface imaging camera  66 . Control lines  32  are indicated by arrows directed toward emitters L, lenses  68 ,  70 , and mirror actuators G-G from the controller. A sense line  34  is indicated by an arrow directed toward the controller from the camera  66 . Each energy beam  63 , 65  may be scanned about two axes by a mirror driven by galvanometers G-G or other means. Alternately, a single pivoting mirror actuator may be provided, with the second dimension provided by a translation mechanism. The third dimension, focus depth, may be controlled by a lens  68 ,  70  to maintain a desired focus of the beam at the workpiece surface  42 . A third laser emitter  38  as in  FIG. 1  (not shown here) may be provided for the camera. Alternately, one or both of the main beams  63 , 65  may be controlled to provide surface image scanning at reduced power to reflect a spot image into the camera for surface analysis. 
         [0020]      FIG. 5  further illustrates a method in which both bending and shrinkage are used to achieve dimensional specifications of the workpiece. A laser bending mechanism such as shortening (see Dearden and Edwardson, supra) may be used to bend the periphery in a direction  72  away from the laser and shorten it  74 . A first laser  62  may scan a beam  63  to heat opposite sides of the bulge periphery or the whole periphery essentially simultaneously. A second laser  64  may scan a second beam  65  over a middle area of the bulge to soften and optionally bend it in a direction  46  toward the emitter with the previously mentioned temperature gradient method. However, if the middle of the bulge is only bent elastically by the distortion, and not plastically, then plastic reversal of the middle of the bulge is not needed. In that case, the two laser devices  62 ,  64  may respectively cover opposite sides of the bulge periphery simultaneously. 
         [0021]      FIG. 6  shows a third embodiment  20 C of an apparatus performing a method of the invention. The apparatus includes a controller  24 , a surface-imaging camera  66 , a controllable laser emitter  76 , and optionally, one or more additional laser emitters  78 . An additional image scanning laser emitter  38  as in  FIG. 1  (not shown here), may be provided for the camera  66 , or the main emitters  76 ,  78  may be controlled for surface imaging. This figure illustrates a method used during repair or fabrication of a component. A gas turbine blade  80  has a pressure side PS, a suction side SS, and a squealer ridge  82  extending above the periphery of a blade tip cap  84 . The squealer ridge on the pressure side is in the process of additive fabrication  88  forming a melt pool  86  of additive superalloy. This process heats the pressure side of the blade tip, and thus distorts the tip by differential thermal expansion as shown by the dashed line  90  in  FIG. 7 . To prevent this, the apparatus of  FIG. 6  detects the distortion early via the scanning camera  66  and/or determines the distortion predictively by mathematical modeling, and applies compensating heating to the suction side of the blade tip. This avoids introducing stress in the blade due to shape changes during processing and cooling of the squealer ridge. It also avoids heating the whole blade in an oven to prevent such distortion during processing and cooling, thus reducing energy and time. 
         [0022]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.