Patent Publication Number: US-2016228995-A1

Title: Material repair process using laser and ultrasound

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of materials technology, and more particularly to processes for the repair of a discontinuity in a substrate material. 
     BACKGROUND OF THE INVENTION 
     Gas turbine hot gas path components are often subject to service-induced degradation in spite of being manufactured from highly durable superalloy materials. The term “superalloy” is used herein as it is commonly used in the art, i.e., a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures. Superalloys typically include a high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g., IN 738, IN 792, IN 939), Rene alloys (e.g., Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM247LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g., CMSX-4) single crystal alloys. 
       FIG. 1  illustrates an exemplary service-induced discontinuity as a crack  10  opening to a surface  12  of a superalloy substrate  14 . A known method of repairing such cracks is laser remelting, as is illustrated in  FIG. 2  where a laser beam  16  is directed to the surface  12  to heat and melt it to form a melt pool  18 . The melt pool  18  encompasses the crack  10 , such that upon removal of the laser beam  16  and cooling and solidification of the melt pool  18 , a renewed surface  20  is formed on the substrate  14 , as illustrated in  FIG. 3 . 
     The known process of  FIGS. 1-3  is not always successful in providing a discontinuity-free surface  20 . As illustrated in  FIG. 3 , artifacts of the laser remelting process may include porosity  22 , inclusions  24  and/or solidification cracks  26 . Such artifacts may result from the presence of contaminants  28  that accumulate in the original crack  10  during service exposure, such as oxides and other foreign debris present in the hot combustion gas of a gas turbine engine. The contaminants  28  mix into the melt pool  18  and may be distributed over a larger volume, but they are not eliminated by the laser remelting process. Pre-melt cleaning of the substrate surface  12  can reduce the quantity of the contaminants  28 , but such cleaning requires advanced and expensive measures such as hydrogen, vacuum or fluoride ion heat treatment. Even after a cleaning process, tight and/or deep cracks are generally incompletely cleaned. 
     Crack prone materials, including superalloys often used in gas turbine engines, are also subject to the formation of cracking  26  as a result of a laser remelting process or a subsequent heat treatment, due to the restraint of the surrounding substrate material as the melt pool  18  cools and shrinks. Certain contaminants  28  can exacerbate this problem. Thus, there continues to be a need for an improved process for repairing a substrate material containing surface and near-surface discontinuities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a cross-sectional illustration of a prior art substrate material containing a surface-opening crack. 
         FIG. 2  illustrates a prior art laser remelting repair process. 
         FIG. 3  illustrates the substrate material of  FIG. 1  after undergoing the laser remelting process of  FIG. 2 . 
         FIG. 4  illustrates a cracked substrate material covered by a layer of powdered material including flux and adjoined to an ultrasonic transducer. 
         FIG. 5  illustrates the substrate material of  FIG. 4  being exposed to laser beam energy and ultrasonic energy to form a melt pool covered by a layer of slag. 
         FIG. 6  illustrates the substrate material of  FIGS. 4 and 5  upon re-solidification of the melt pool and layer of slag. 
         FIG. 7  illustrates the substrate material of  FIGS. 4-6  after removal of the layer of slag to reveal a renewed surface having no crack or other discontinuity. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors have developed a hybrid process for repairing a material substrate which contains a discontinuity, such as a surface or subsurface crack, pit, inclusion, void, porosity, or other off-design condition. This process applies both an energy beam and vibratory mechanical energy in the region of the discontinuity in order to produce a renewed substrate surface free of the discontinuity and less susceptible to undesirable repair artifacts than can be achieved with prior art laser remelting processes. The utilization of both an energy beam and vibratory mechanical energy can improve the removal of harmful contaminants present in the discontinuity, can improve the control of the introduction of heat energy into the repaired material, and can reduce residual stresses in the substrate material resulting from the repair process. 
       FIGS. 4-7  illustrate an embodiment of the invention. A substrate  30  contains a surface  32  containing a discontinuity such as a crack  34 , such as a service-induced crack in a superalloy component of a gas turbine engine, as shown in  FIG. 4 . The crack  34  may contain contaminants which are difficult or impossible to remove with known cleaning processes. In this embodiment, a layer of powdered material  36  is placed onto the surface  32  over the crack  34 . The powdered material  36  includes a flux material, but in other embodiments may include or be only an alloy filler material, as more fully described below. An electro/mechanical transducer  38  is positioned in contact with the substrate  30  at a location adequate for the introduction of vibratory mechanical energy into the substrate  30  proximate the crack  34 . 
       FIG. 5  illustrates the substrate  30  of  FIG. 4  being exposed simultaneously to both a laser beam  40  (source not illustrated) and mechanical vibratory energy  42  produced by the transducer  38 . While illustrated as a laser beam  40  in  FIG. 5 , other embodiments of the invention may utilize another type of beam energy, such as an ion beam, electron beam, etc. The mechanical vibratory energy  42  may be of any or varying frequencies, and in one embodiment is ultrasonic energy. The combined effect of the laser beam  40  and mechanical vibratory energy  42  is melting of the substrate  30  surrounding the crack  34  and melting of the overlying powdered material  36 , thereby producing a melt pool  44  and, for the embodiment of powdered flux material  36 , an overlying layer of slag material  46 . As taught in commonly assigned United States patent application publication number US 2013/0136868 A1, incorporated by reference herein, flux material is advantageously effective to trap laser energy, provide atmospheric shielding, cleanse contaminants, control cooling, and optionally to provide a material additive function, making it particularly useful for the repair of difficult to weld superalloy materials. 
       FIG. 6  illustrates the substrate  30  after cooling and solidification of the melt pool  44  and layer of slag material  46 , and  FIG. 7  illustrates the substrate  30  after removal of the slag material  46 , revealing a renewed surface  48  free of any discontinuity. 
     The application of vibratory mechanical energy  42  during the formation of the melt pool  44  in  FIG. 5  provides agitation which can promote mixing, agglomeration and floatation of contaminants captured in the slag. The vibratory mechanical energy  42  may also or alternatively be applied before the formation of the melt pool  44 , such as in the step of  FIG. 4 , in order to dislodge contaminants within the crack  34  and/or to create heat within the crack  34  via friction between opposing sides of the crack  34 . The vibratory mechanical energy  42  may also or alternatively be applied after the formation of the melt pool  44 , such as in the step of  FIG. 6 , in order to dislodge the layer of slag  46  and/or to provide a vibratory stress relief function. 
     Flux material may be applied over the crack  34  in powder, paste, liquid or foil form, and it may be preplaced, as shown in  FIG. 4 , or it may be applied concurrently with the application of the beam energy with a known feeder system. The flux may contain an additive constituent which alloys into the melt pool  44  to achieve a desired material composition or to compensate for a material that is lost as a result of the beam melting process, for example titanium or aluminum. A filler material powder may be included with the flux, the filler material powder contributing to the melt pool in order to add volume to compensate for discontinuity voids or to alter the chemical composition of the melt pool. 
     In one embodiment, a flux material is introduced into the discontinuity in the form of a liquid or paste. Beam energy is then applied to pre-heat the substrate material to a temperature close to but below a melting point of the substrate material. Mechanical vibratory energy is then applied to dislodge contaminants within the discontinuity and to create additional heat within the discontinuity due to friction, resulting in the formation of a small melt pool immediately around the discontinuity. The flux then functions to float the contaminants out of the melt pool as slag, which is then removed upon cooling and re-solidification of the melt pool. 
     It may be advantageous in this or other embodiments for the flux to include a composition that becomes exothermic when melted in order to further enhance and control the heating process. The exothermic agent may be any substance that undergoes a chemical reaction to produce heat. In some embodiments the exothermic agent is metal, metal alloy or metal composition which reacts with oxygen to produce heat. One example of such a reaction is the combustion of zirconium metal with oxygen to form zirconium oxide as shown below in equation (A): 
       Zr(s)+O 2 →ZrO 2 (s)  (A)
 
     Other examples of similar exothermic reactions which may be useful for specific applications include: 
       Fe 2 O 3 +2Al→2Fe+Al 2 O 3 (iron thermite)  (B)
 
       3CuO+3Al→3Cu+Al 2 O 3 (copper thermite)  (C)
 
     In another embodiment, a powder, liquid, paste or foil material is applied over the surface in a region of a discontinuity, and both mechanical vibratory energy and an energy beam are then applied to the substrate in the region of the discontinuity to melt and to distribute the applied material. The melted material is then allowed to solidify to from a repaired surface on the substrate. 
     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.