Abstract:
A superalloy component, such as gas turbine blade or vane, is structurally welded by placing the component in an isolation chamber. Inert gas is introduced into the chamber. The substrate is welded in the chamber, creating a weld zone. Pressure is applied directly on the weld zone that is greater than atmospheric pressure. Application of such pressure increases the weld zone ductility and reduces likelihood of solidification cracking and strain age cracking, compared to weld zones formed at atmospheric pressure. In some embodiments an isostatic pressure chamber is used to apply isostatic pressure on the weld zone. In other embodiments the welding is performed by laser welding or cladding, TIG welding electron beam welding or autogenous welding.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application incorporates by reference in its entirety commonly owned, co-pending United States Utility patent application entitled “SUPERALLOY LASER CLADDING WITH SURFACE TOPOLOGY ENERGY TRANSFER COMPENSATION” filed Sep. 12, 2012 and assigned Ser. No. 13/611,034. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to methods and apparatus for welding and cladding superalloy materials, typically components such as service-degraded turbine blades and vanes, while reducing or avoiding cracking of the material and improving its ductility. More particularly, the present invention relates to method and apparatus for welding or clading high γ′ Ni based superalloys with a reduction or avoidance of cracking and an improved ductility by the application of elevated pressure. 
         [0004]    2. Description of the Prior Art 
         [0005]    “Structural” repair of gas turbine or other superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving properties, such as strength, that are close to the original manufacture component specifications (e.g., at least seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced surface cracks, so that risk of further cracking is reduced, and the blades are restored to original material structural and dimensional specifications. 
         [0006]    Structural repair of high γ′ nickel based superalloy material that is used to manufacture turbine components, such as turbine blades, is challenging, due to the metallurgic properties of the finished blade material. Superalloys, such as N5, CM247, MARM002, IN738, among others, are often used in gas turbine applications, due to their good high temperature environment and mechanical properties. The aluminum and titanium content of the superalloys, such as N5 and CM247 (among others) are increased to obtain higher mechanical strength and heat resistance properties, and the finished turbine blade alloys are typically further strengthened during post casting heat treatments. A superalloy having more than about 6% aggregate aluminum or titanium content, such as CM247 or N5 alloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content superalloy such as X-750. The weldability and reparability of these superalloy materials is decreased due to hot cracking, grain boundary embrittlement, solidification cracking and strain age cracking. These N5 and CM247 superalloy materials possess a “zero ductility range” when cooled from weld temperature. Most of the cracking occurs in the “zero ductility range”. Due to the loss of ductility, CM247 and N5 superalloy components cannot be structurally repaired to achieve the desired 70% or greater original specification tensile strength by traditional welding processes with commercially acceptable repair yield results. 
         [0007]    Traditional welding processes for superalloy fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding rod or other filler material added. When a blade constructed of such a material is welded with filler metal of the same or similar alloy, the blade is susceptible to solidification cracking (aka liquation cracking) within and proximate to the weld, and/or strain age cracking (aka reheat cracking) during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties to those comparable to a new component. Low successful structural repair yield rates by traditional welding processes make it commercially cheaper to scrap cracked blades and vanes that need structural repair rather than waste effort to repair them. 
         [0008]    As an alternative to traditional superalloy component turbine blade and vane structural welding processes, another known superalloy joining and repair method that attempts to melt superalloy filler material without thermally degrading the underlying superalloy substrate is laser beam welding, also known as laser beam micro cladding or selective laser melting (SLM). Superalloy filler material (often powdered filler) compatible with or identical to the superalloy substrate material is pre-positioned on a substrate surface or sprayed on the surface during the cladding process. A “spot” area of focused laser optical energy generated by a fixed-optic laser (i.e., other than relative translation, laser and substrate have a fixed relative orientation during laser beam application) liquefies the filler material and heats the substrate surface sufficiently to facilitate good coalescence of the filler and substrate material, that subsequently solidify as a clad deposit layer on the substrate surface. Compared to other known traditional welding or cladding processes, laser beam micro-cladding is a lower heat input process, with relatively good control over melting of the substrate and rapid solidification that reduce propensity to cause previously-described solidification cracking. Lower heat input to the superalloy substrate during laser welding/cladding also reduces residual stresses that otherwise would be susceptible to previously described post-weld heat treatment strain age cracking. While laser cladding welds have structural advantages over traditionally-formed welds, practical manufacturing and repair realities require larger cladding surface area and/or volume coverage than can be filled by a single-pass applied cladding deposit. 
         [0009]    In the previously identified, incorporated by reference, copending application Ser. No. 13/611,034, a superalloy substrate, such as a turbine blade or vane, is fabricated or repaired by laser beam welding to clad one or more layers on the substrate. Laser optical energy is transferred to the welding filler material and underlying substrate to assure filler melting and adequate substrate surface wetting for good fusion. Energy transfer is maintained below a level that threatens thermal degradation of the substrate. Optical energy transfer to the filler and substrate is maintained uniformly as the laser beam and substrate are moved relative to each other along a translation path by varying the energy transfer rate to compensate for localized substrate topology variations. 
         [0010]    Thus, a need exists for welding/cladding processes, materials and apparatus for welding/cladding superalloys with reduced occurrence of cracking, either during the welding/cladding process or during post-processing heat treatment. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention relates generally to welding/cladding a superalloy component, such as a gas turbine blade or vane, with the application of elevated pressure. Some embodiments relate to laser welding/cladding in an isostatic pressure chamber in an intet gas atmosphere with pressure applied during the welding/cladding process as well as during the subsequent cooling. The applied isostatic pressure inhibits cracking during welding/cladding and subsequent cooling, apparently by the mechanism of compressing the component and its weld zone to counteract tensile stresses developing in the weld zone. Preferably, sufficient isostatic pressure is applied to increase weld zone ductility to be greater than ductility of a comparable weld zone formed at atmospheric pressure. 
         [0012]    An exemplary embodiment method for cladding superalloy components features placing a superalloy component substrate in an isostatic pressure chamber. Filler material, which may comprise the same superalloy material as the component, is typically introduced on the substrate surface. Inert gas is introduced into the pressure chamber. Isostatic pressure is applied in the pressure chamber on the filler material and substrate that is greater than atmospheric pressure. A laser beam is focused on the filler material and substrate. The laser transfers optical energy to the filler material and substrate in a weld zone that fuses the filler material to the substrate. In some embodiments sufficient isostatic pressure is applied to increase weld zone ductility compared to fusion at atmospheric pressure. Isostatic pressure up to 100 ksi (689.4 kPa) may be applied in the pressure chamber. In some embodiments the inert gas comprises argon or nitrogen. The pressure chamber may be heated before fusing the filler material to the substrate. In some embodiments the substrate and laser beam are moved relative to each other to form a multi-dimensional weld zone. 
         [0013]    Other embodiments include welding without the addition of filler material, or autogenous welding as well as other means for the application of pressure. Pressure may be applied during or immediately after the welding process. 
         [0014]    Another exemplary embodiment features a turbine component selected from the group consisting of turbine blades and turbine vanes, having a superalloy substrate and crack-free weld zone having a first ductility by placing a superalloy component substrate in an isostatic pressure chamber, introducing superalloy filler material on a component superalloy substrate surface and introducing inert gas in the pressure chamber. Isostatic pressure is applied in the pressure chamber on the superalloy filler material and substrate that is greater than atmospheric pressure. A laser beam is focused on the filler material and substrate. Optical energy from the laser beam is transferred to the superalloy filler material and substrate in a weld zone that fuses the filler material to the substrate in the weld zone. In some embodiments, the applied isostatic pressure causes the first ductility to be greater than ductility of a weld zone formed at atmospheric pressure. Isostatic pressure up to 100 ksi (689.4 kPa) may be applied in the pressure chamber. The pressure chamber may be heated before fusing the filler material to the substrate. In some embodiments the inert gas comprises argon or nitrogen. In some embodiments the substrate and laser beam are moved relative to each other to form a multi-dimensional weld zone. 
         [0015]    Another exemplary embodiment features a system for cladding turbine superalloy components with a superalloy filler layer. A laser generates a laser beam for transferring optical energy to a turbine component superalloy substrate and superalloy filler material on the substrate that fuses the filler material to the substrate in a weld zone, forming the filler layer. The system also has an isostatic pressure chamber for retaining inert gas, the substrate and filler material, and for generating isostatic pressure that is greater than atmospheric pressure during weld zone formation. In some embodiments the pressure chamber has a sealed window for transmitting the laser beam there through from outside the chamber to inside the chamber. In some embodiments the pressure chamber is a heated pressure chamber and/or is capable of applying isostatic pressure up to 100 ksi (689.4 kPa). In some embodiments the system also has a motion control system for moving the substrate and laser beam relative to each other to form a multi-dimensional weld zone. The motion control system may comprise at least one movable mirror intercepting the laser beam, for orienting the laser beam on the substrate; and at least one drive system coupled to each of the respective at least one movable mirror and the laser for causing relative motion between the laser beam and substrate, and for maintaining uniform energy transfer to the substrate. The laser and motion control system may comprise a galvanometer scanner 
         [0016]    The features of the present invention may be applied jointly or severally in any combination or sub-combination by those skilled in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  shows a schematic perspective view of a first exemplary embodiment of an isostatic pressure welding apparatus of the present invention; 
           [0019]      FIG. 2  shows a schematic perspective view of a second exemplary embodiment of an isostatic pressure welding apparatus of the present invention; 
           [0020]      FIG. 3  shows a schematic perspective view of a third exemplary embodiment of an isostatic pressure welding apparatus of the present invention; and 
           [0021]      FIG. 4  shows a schematic perspective view of a fourth exemplary embodiment of an isostatic pressure welding apparatus of the present invention. 
       
    
    
       [0022]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
       DETAILED DESCRIPTION 
       [0023]    After considering the following description, those skilled in the art will clearly realize that the teachings of the present invention can be readily utilized in welding/clading of superalloy components, such as gas turbine blades and vanes, including structural welding of such superalloy components. Some embodiments of the present invention elate to an isostatic pressure laser welding apparatus and method to facilitate crack-free structural welding of superalloy components with superalloy filler material. The apparatus and method embodiments of the present invention inhibit solidification and post-weld heat treatment cracking of the superalloy components in the weld zone. 
         [0024]    As previously noted, as the aluminum and titanium content of the superalloys is increased to obtain better mechanical properties; their weldability and reparability is decreased due to hot cracking, grain boundary embrittlement, solidification cracking and strain age cracking. Crack formation in materials is attributable to void formation or interface opening under tensile stress. Low ductility in a material renders it more susceptible to crack formation. Turbine blade and vane components formed from superalloys, such as N5 and CM247, possess a “ductility dip range”. when a weld zone therein is cooled from weld temperature. Most of the cracking occurs in the zero or low ductility weld zone. Generally, a material&#39;s ductility can be increased by applying hydrostatic pressure on the material. By applying isostatic pressure to the material, ductility can be increased threefold compared to ductility at atmospheric pressure. As noted, crack formation in materials is attributable to void formation or interface opening under tensile stress. Application of a compressive hydrostatic pressure on the material counteracts the tensile stress and reduces likelihood of crack formation. Thus application of compressive hydrostatic pressure, such as isostatic pressure, provides a dual benefit of counteracting tensile stresses that promote crack formation and increasing the material ductility (i.e., decreasing crack formation susceptibility). Ductility increase does not negatively impact material strength as strength is not affected by hydrostatic/isostatic pressure. 
         [0025]    In situ compressive stresses are applied on superalloy component weldments during or immediately after welding and while cooling from welding temperature, in order to inhibit microcracking in the weld zone and enhance weld zone ductility. The compressive stresses are advantageously applied in isostatic pressure chambers, however pressure applied by means of direct contact loads during welding can also be employed. Thus the external pressure application at levels greater than atmospheric pressure inhibits void formation and crack opening during welding and cooling, while providing additional benefit of increasing weld zone ductility. As described in greater detail below, embodiments utilize laser welding in a hydrostatic and/or isostatic pressure chamber to achieve crack-free superalloy laser structural weld build up, laser cladding and welding. Laser heating of the substrate and filler material is conducted under inert gas atmosphere at a maximum pressure of approximately 100 ksi (689.4 kPa). The pressure chamber may be heated depending on the alloy type further to improve material ductility. 
         [0026]    To be concrete in our description, we focus on the particular embodiments using laser welding/cladding , including a filler material, and isostatic application of pressure. Other embodiments within the scope of the present invention include other means for the application of pressure, such as direct contact loading, peening, and rolling. In other embodiments there is an absence of filler material in autogeneous welding processes. Other types of welding processes are used in other embodiments, such as electron beam welding or Tungsten-Inert-Gas (TIG) welding. 
         [0027]    Furthermore, our focus herein is on the description of laser welding processes, but the present invention is not limited to typical welding. Surface cladding with the addition of powder, sheet or foil cladding materials are included within the scope of this invention as would be apparent to those having ordinary skills in the art. 
         [0028]    A first embodiment of the present invention is shown in  FIG. 1 . A superalloy component substrate  10 , such as a turbine blade or vane, has a weld zone  12  formed by the heating of the substrate and filler material  14 . Often the filler material  14  is applied to the component substrate  10  prior to commencement of the welding operation. Alternatively filler material  14  may be applied with a filler spreader solely or in conjunction with pre-applied filler, using known application techniques. The filler material  14  may comprise powdered superalloy material, and may comprise the same material as the component substrate  10 . Weld zones  12  that are formed with superalloy filler material  14  may constitute structural welds of the component substrate  10 . The component substrate  10  is placed on a work piece fixture system  20  that optionally may include a work table  22  and motion control system  24  for imparting desired relative motion between the substrate  10  and a laser welding apparatus  30 . The laser welding apparatus  30  focuses a laser beam  32  on the filler material  14  and substrate  10 , transferring optical energy to both, and forming the weld zone  12  that fuses the filler material to the substrate. Optionally the laser welding apparatus  30  includes a motion control system  34  for imparting desired relative motion between the substrate  10  and the laser beam  32 . Relative motion between the laser beam  32  and the substrate  10  forms a multi-dimensional weld zone. 
         [0029]    Optionally, a control system can be included ( 170  in  FIG. 4 ) for changing the characteristics of the delivered laser energy during processing. Typically, the laser energy and/or spot size may be altered during processing to adjust for changing contours of the component substrate  10 , changing applied pressure, among other factors. 
         [0030]    The superalloy component substrate  10  and the filler material  14  are placed and retained within an isostatic pressure chamber  40  in an inert gas environment. Suitable inert gasses include argon and nitrogen. Isostatic pressure is applied in the pressure chamber  40  prior to commencement of welding through component cooling. Pressure level Δ P  is applied within the pressure chamber  40  at a sufficient level greater than atmospheric pressure to inhibit formation of cracks in the weld zone  12  and desirably may be applied at a sufficient level to increase ductility of the component weld zone and component substrate  10 , up to approximately 100 ksi (689.4 kPa). The pressure chamber  40  may be heated to a temperature Δ T  (below the lower of the component  10  or filler material  14  solution temperatures), depending on the alloy type , further to improve substrate  10  and weld zone  12  material ductility. State variables (temperature Δ T  and pressure Δ P ) are desirably used in conjunction with laser welding to increase the weld zone  12  and/or component substrate  10  material ductility from a “zero ductility range” to a weldable ductility range. 
         [0031]    Laser beam transparent window  42  allows laser light transmission and is incorporated in the pressure chamber  40 . The window  42  may be constructed with a single crystal sapphire (AI203). Sapphire is anisotropic and has hexagonal closed packed crystalline structure. The orientation of sapphire window  42  to the laser beam  32  is either parallel or perpendicular to its crystalline “c” axis. Any window with sufficient mechanical properties may be substituted for one constructed of sapphire. Location of window  42  is variable. It can be located at the bottom of the chamber  40  and the laser beam  32  can be directed with optical mirrors to the weld. 
         [0032]    In the embodiment of  FIG. 2  the laser welding apparatus  30  is retained within the pressure chamber  40 , eliminating the need for a transparent window. Otherwise the embodiments of  FIGS. 1 and 2  have the same types of components and operation. 
         [0033]      FIGS. 3 and 4  are respective additional embodiments corresponding to those of  FIGS. 1 and 2 , that incorporate the multi-dimensional surface topology laser welding apparatus and methods of the commonly owned, incorporated by reference U.S. application Ser. No. 13/611,034 for the work piece fixture system  20  and laser welding apparatus  30  of  FIGS. 1 and 2 . In  FIG. 3  the laser is oriented outside the pressure chamber  40  and the laser beam is transmitted through the window  42 . Conversely in  FIG. 4  the laser is oriented within the pressure chamber  40 . 
         [0034]    With reference to either of  FIG. 3  or  4 , laser welding/cladding system  100  includes a work table  120  to which is affixed a work piece such as turbine blade  10 . Optional work table motion control system  125  is used to move the work table  120  in the X, Y and Z coordinates shown or in any other single- or multi-axis coordinate system. A known powdered filler hopper  130  with application valve  135  introduces powdered filler material  136  to the turbine blade  10  substrate surfaces. The powder filler may have its own independent motion control system for moving the poured powder  136  application zones relative to the blade  10  substrate. Powdered filler material  136  may be fed to the fixed point of beam focus, as shown, off-axis powder feeders can spray filler powder over the full translational range of laser beam scanning, and elongated powder feeder nozzles can be employed. Powder  136  may also be pre-placed on the substrate  10  before the laser welding process is initiated. Solid wire or strip filler may be substituted for powder filler material  136 . 
         [0035]    The system  100  has a laser  140  with optional variable focus dF or power output dP that provides the laser beam optical energy source for heating the substrate  10  and filler material  136 . The system  100  also has a moveable mirror system  150  with mirror  160  that is capable of single- or multi-axis movement, shown as tilt T, pan P and rotate R axes under control of respective drives  162 ,  164  and  166 . The drives  162 ,  164  and  166  may be part of a known construction motorized motion control system or incorporated in a known galvanometer, that are under control of known controller  170 . 
         [0036]    Alternately the beam may be intercepted by multiple mirrors with single (or multiple) axes of motion to achieve each of the afore-described axes movements. 
         [0037]    The controller  170  may be a stand-alone controller, programmable logic controller or personal computer. The controller  170  may also control one or more of the work table motion control system  125 , the powdered filler material hopper valve  135  and/or the optional hopper motion control system (not shown), and/or the laser  140  variable focus dF and/or power output dP. Known open and/or closed feedback loops with the controller may be associated with one or more of the drives  125 ,  135 ,  162 - 166 , dF, dP, and the hopper position drive. Laser beam optical energy transfer to the substrate and filler can also be monitored in a closed feedback loop so that the controller can vary the energy transfer rate based on the monitored energy transfer rate. A human machine interface (HMI) may be coupled to the controller  170  for monitoring welding operations and/or providing instructions for performing a welding operation. 
         [0038]    When operating the welding system  100  the output beam  180  of the laser  140  is reflected off mirror  160  (or multiple mirrors) and in turn on to the turbine blade  10  work piece, which transfers optical energy to the turbine blade  10  and filler material  136 . Both the turbine blade substrate  10  and filler material  136  absorb the transferred optical energy, to melt the filler material, wet the substrate surface and fuse the melted filler and substrate surface to each other. The substrate  10  and laser beam  180  are moved relative to each other along a translation path by the control system engagement of the work table drive system  125  and/or the moveable mirror system  150  drives  162 ,  164 ,  166  to form a continuous welded cladding layer  200 . When the movable mirror system  150  is incorporated in a commercially available laser galvanometer system, relative motion between the substrate  10  and the laser beam  180  as well as the laser optical energy transfer rate can be varied by moving the galvanometer mirror  160  (or multiple mirrors) for both relative translation and oscillation. Relative motion between the laser beam  180  and the substrate  10 /filler material  136  maintains a continuous melted weld line at the leading edge of translation motion (e.g., the right leading edge of the weld line in  FIGS. 3  or  4 ) for fusion uniformity that is not possible with known unoscillated laser cladding systems. 
         [0039]    The laser optical energy absorbed at any beam focus area varies proportionately with focus time duration. By non-limiting example laser beam  180  focus time duration and proportional absorbed energy can be varied in the following ways: (i) the laser beam  180  can be oscillated parallel to or side-to-side transverse (e.g.,  211 ) the weld translation path  210 ; (ii) the oscillation or translation speed can be varied; and (iii) the laser power intensity dP or focus dF can be varied continuously or by pulse modulation. Thus by dynamically varying changing the rate of laser beam focus time duration the energy transfer rate to the substrate and filler is varied along the weld line translation path, so that uniform energy transfer is maintained within the entire weld, regardless of local topography variations. 
         [0040]    Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. The invention is not limited in its application to the exemplary embodiment details of construction and the arrangement of components set forth in the description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.