Abstract:
A method of welding including: applying a flux having at least a majority weight percent boron to a surface of a superalloy base material; forming a weldment on the surface wherein boron is melted onto the surface and is incorporated into a resulting weld pool and heat affected zone, and wherein incipient melted inter-dendritic material resulting from presence of the boron is available to flow into a crack formed during cooling of the weldment; and heat treating the weldment to diffuse a remaining concentration of the boron in the weldment and heat affected zone to a desired value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/332,561 filed May 6, 2016, the disclosure of which is hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of welding of superalloys that heals weld-induced cracks. 
       BACKGROUND OF THE INVENTION 
       [0003]    Highly alloyed nickel and cobalt castings (e.g. CM-247 LC®, Inconel®-738, GTD-111™, MGA-1400, ECY-768, MAR-M 509® etc.) are commonly used in gas turbine engine hot gas path applications. Alloying elements (e.g. Al, W, C, Ti, Ta) used in the castings increase the difficulty of achieving good castings and reduce the weldability of components made of the castings. In particular, the presence of these alloying elements may lead to cracking in the weld and heat affected zone (HAZ) of the casting when welded. However, welding can be a necessary part of fabrication and/or repair of these components. To achieve crack free weldments, one approach has been to use relatively ductile weld fillers (e.g. Inconel®-625, Haynes®-230®, Haynes®-188, Nimonie-263, Inconel®-617, Merl-72, Waspaloy®, etc.). These fillers have a reduced mechanical strength and oxidation resistance compared to the nickel and cobalt castings (i.e. base metals) where operating temperatures exceed 1800 degrees Fahrenheit. Consequently, these ductile weld fillers cannot be used in some applications. 
         [0004]    It is known to use boron as a melting point suppressant in welding. U.S. Pat. No. 2,507,751 to Bennett discloses using a slag-forming flux containing a minority amount of boron for improved wetting action and lowered surface tension. Bennett cautions against using too high of a percentage of boron. United States Patent Application Publication No. US 2015/0298263 A1 to Goncharov, et al. discloses a welding wire having a coating containing less than 10% boron and silicon. Blacksmiths have been known to take steel up to orange color, apply boron, take the steel up to yellow color, and tap the steel onto itself to incorporate the boron. However, in that process no material is melted and the boron is understood to act on the base metal as a whole. 
         [0005]    There remains room in the art for improvement with respect to welding high alloy materials such as modern superalloys. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is explained in the following description in view of the drawings that show: 
           [0007]      FIG. 1  is a schematic representation of the welding process disclosed herein. 
           [0008]      FIG. 2  is a schematic representation of dendritic grain structure with an inter-dendritic region before boron infusion. 
           [0009]      FIG. 3  is a schematic representation of the dendritic grain structure with an inter-dendritic region of  FIG. 2  after boron infusion and showing incipient melting. 
           [0010]      FIG. 4  is a schematic representation of the dendritic grain structure with an inter-dendritic region of  FIG. 3  showing crack formation and the crack healing process. 
           [0011]      FIG. 5  is a schematic representation of the dendritic grain structure with an inter-dendritic region of  FIG. 4  showing healed cracks. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The present inventors have devised a unique method of welding a nickel, cobalt, or iron based superalloy that enables the use of a base material equivalent weld filler material with reduced cracking. As used herein a base material equivalent is one recognized by those of ordinary skill in the art as having the same or essentially the same chemical composition as a base material. The method includes applying essentially pure boron to a cast superalloy component proximate the location where the weld is to be formed, and then forming the weld. The boron melts in advance of the moving weld pool and functions to shield the heated, but still solid, material. Boron is then incorporated into the weld pool and also diffuses into the heat affected zone (HAZ) of the cast superalloy component. The boron lowers the melting point of material in interdendritic zones of the cast superalloy component, which contributes to incipient melting in the interdendritic zones. If a crack forms, incipiently melted material in the interdendritic zone can flow into the crack, thereby healing the crack. The flow of incipiently melted material may be aided by a vacuum created within the crack as a result of the crack formation which draws the incipiently melted material into the crack. The lower melting point of some material in the weld pool allows the lower melting temperature material to flow more readily throughout higher melting temperature material as the higher melting material solidifies and changes volume. This provides a degree of conformity as the weld cools and solidifies, thereby reducing crack formation in the weld as well. 
         [0013]      FIG. 1  is a schematic representation of an exemplary embodiment of the welding process disclosed herein. An incipient melt facilitator  16  in the form of a flux paste  12  is preplaced directly on a superalloy substrate  14 . The paste  12  may be applied by any number of methods, including but not limited to simply brushing the paste  12  on the component  10 . The paste may be applied to in a thickness, e.g., in the range of (0.005″-0.020″). The paste  12  includes an incipient melt facilitator  16  that locally reduces a melting temperature of at least a portion of the superalloy substrate  14 . This reduces the amount of heat that must be input during the process, which reduces heat related problems such as distortion and cracking, etc. An example of such an incipient melt facilitator  16  is boron, and the paste  12  may include boron and a carrier such as alcohol. The boron may be amorphous or it may have an identifiable crystal shape. The boron may be anhydrous or it may include water. Unlike prior art welding fluxes typically used for superalloy materials, the paste may be composed of greater than 50% by weight of boron, or greater than 75% or 90% or 99% by weight boron in various embodiments. In an exemplary embodiment, the paste may be borax or any allotrope of boron. The borax may include Na 2 B 4 O 7  with or without water. The paste  12  may be spread to any width  18  about the weld joint line. In an exemplary embodiment, the width  18  may be sufficient to cover a weld bead  20  and a heat affected zone  22  with overage  24  extending past the heat affected zone  22  to ensure adequate coverage. However, the width  18  may also be narrower. 
         [0014]    The weld bead  20  may be formed by heating the substrate  14  and a weld filler material  30  via an energy beam  32  generated by an energy beam source  34 . The substrate  14  may be a nickel, cobalt, or iron based superalloy. The weld filler material  30  may be a filler powder  36  that is preplaced on, under, or mixed in with the paste  12 . Alternately, or in addition, the weld filler material  30  may be delivered directly to a melt pool  40  via a delivery arrangement  42 . The weld filler material  30  may be solid, for example, in rod form. 
         [0015]    The weld filler material  30  may be a material that has the same composition as the superalloy of the substrate  14 , or a similar composition which after welding forms a base material equivalent weld deposit  20 . Alternately, the weld filler material  30  may include a material that is superior to the superalloy of the substrate  14  in some desired functionality but that otherwise has been found to be difficult to deposit without cracking prior to the present invention. Alternately, the welding process may use no weld filler material  30  (e.g. autogenous). The energy beam may be a laser beam or an electron beam, although other methods of heat delivery may be used, with or without preheating. The process may occur under the protection of a shielding gas, such as an inert shielding gas. Alternately, the weld may instead be formed via other processes such as gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), metal inert gas (MIG), metal active gas (MAG), plasma arc welding (PAW), submerged arc welding (SAW), friction stir welding, and their derivatives. 
         [0016]    In the process of  FIG. 1 , as the melt pool  40  moves in a direction of travel  44 , some or all of the paste  12  is consumed. The paste  12  may become molten ahead of the melt pool  40 , which may provide additional atmospheric protection to the heat affected zone  22 . Some of the incipient melt facilitator  16  (e.g. boron) in the paste  12  is incorporated into the melt pool  40 , and some of the incorporated incipient melt facilitator  16  diffuses into the heat affected zone  22 . Incipient melt facilitator  16  in the paste atop the heat affected zone  22  may diffuse directly into the component  10 . The incipient melt facilitator  16  that has diffused into the heat affected zone  22  suppresses a melting temperature of at least one part of the material of the heat affected zone  22  such that some of the material in the heat affected zone  22  experiences incipient melting. Any cracks or fissures that form in an unmelted portion of the heat affected zone  22  during cooling may be filled (e.g. healed) by the incipiently melted material in the heat affected zone  22  that can flow into the crack. Likewise, relatively low melting temperature material in the melt pool  40  can conform to previously solidified material as the weld bead  20  solidifies and takes shape, thereby reducing cracking in the weld bead  20 . 
         [0017]    A welding process using superalloy filler material and flux material is disclosed in United States Patent Application Publication No. 2013/0136868 A1 to Bruck et al., and is incorporated herein by reference. The present invention may be used with such a welding process to weld a superalloy using powdered superalloy weld filler material, powdered flux material, and the incipient melt facilitator  16  disclosed herein. These materials may be applied in discrete layers in any order, or some or all of them may be blended as desired. It should be appreciated that weld parameters for reduced heat input welding enabled by boron may include, e.g., a filler metal diameter of (0.035″-0.092″), Current Type &amp; Polarity (DC Straight-AC), Amps (5-210), and a travel speed of (½Inch/min-20 inch/min). 
         [0018]    With continued reference to the figures,  FIG. 2  shows a dendritic structure  50  of a crystal  52  of the superalloy component  10  in the heat affected zone  22 . Each dendrite  54  includes a trunk  56  and branches  58 . An interdendritic region  60  exists between the dendrites  54  and includes segregates  62  of different alloying elements (e.g. Ti and Al). This includes gamma prime (γ′) phases with different chemical compositions at various regions within the microstructure, which leads to different melting temperatures. In addition, most superalloys include other phases such as gamma-gamma prime (γ-γ′) eutectics, MC carbides (where M represents one or more metallic atoms), topologically closely packed (TCP) phases, eta (η) phase, and borides. Variations of any of these may vary the local melting temperature, and areas with relatively low melting temperatures may experience the incipient melting described herein. 
         [0019]    The segregates  62  may include the eta (η) phase  64  in the form of plates  66  and other segregates  68  in between the plates  66 . In the exemplary embodiment the incipient melt facilitator  16  is boron, and in  FIG. 2  the boron  16  is illustrated as being at the beginning stages of its diffusion process into the interdendritic region  60  in the heat affected zone  22 . As the boron diffuses, it reaches the regions of differing chemical composition and suppresses respective melting temperatures further, thereby promoting incipient melting. 
         [0020]      FIG. 3  shows the boron diffused into the interdendritic region  60  of the heat affected zone during the welding process as indicated by the letter “B.” The boron has suppressed the melting temperature locally and incipient melting has occurred, creating incipiently melted material  70  in the interdendritic region  60  as indicated by the wavy lines. In this exemplary embodiment, the dendritic structure  50  remains solid. 
         [0021]    In  FIG. 4  an interdendritic region crack  72  and a dendritic crack  74  have formed during cooling. Incipiently melted material  70  proximate the respective cracks  72 ,  74  can flow into the cracks  72 ,  74 . This flow may be aided by a vacuum created in the cracks  72 ,  74  when the cracks  72 ,  74  form. The incipiently melted material  70  may flow into some or all of a respective volume of each crack  72 ,  74 , thereby healing the cracks  72 ,  74 . 
         [0022]      FIG. 5  shows the interdendritic region  60  after all materials have solidified, including healed cracks  76  filled with solidified incipiently melted material. During this process, and even thereafter at elevated temperatures, the boron continues to diffuse over time such that its suppressive and palliative effect ceases. The regions of differing chemical composition then return to their respective unsuppressed melting temperatures. The rate of this diffusion can be accelerated by a conventional post weld heat treatment (PWHT). 
         [0023]    From the foregoing it can be seen that using an incipient melt facilitator, such as boron, can heal cracks during a welding process for a material such as a difficult to weld superalloy. This increases production yield previously lowered by weld induced cracks. Advantageously, the application of a boron paste directly onto the weld joint allows the use of base metal equivalent weld filler materials. Borax or other forms of boron are inexpensive, such as about $2/pound as compared to perhaps $50/pound for typical weld grade flux materials. 
         [0024]    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.