Abstract:
A flux material that provides a heat outflow control layer of slag ( 30 ) on a melt pool ( 20 ) that suppresses lateral heat outflow ( 27 ) and facilitates uniaxial heat outflow ( 26 A-D) from the melt pool at a rate that causes unidirectional crystallization in the melt pool to match a crystal direction ( 24 ) of a substrate ( 22 ). The slag may be insulative, and may flow to form a greater slag thickness (T 2,  T 3 ) at the sides of the melt pool than at the middle (T 1 ). The flux may contain constituents that warm the sides of the melt pool by exothermic reaction. The flux may be used in combination with insulating elements ( 32 A-B,  38 A-B,  44 ) placed on the substrate surface beside the melt pool and/or with supplemental heating of the sides of the weld.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of metals joining, and more particularly to flux materials and welding methods useful for directionally solidified components such as single crystal superalloy gas turbine engine airfoils. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas turbine engines operate more effectively at higher temperatures. An increase of 56 degrees Celsius in a turbine&#39;s firing temperature can provide a corresponding increase of 8-13% in output and 2-4% of improvement in simple cycle efficiency. Therefore, advanced gas turbines use directionally solidified materials including single crystal superalloys with high temperature performance including creep resistance. Stray grains can form during single crystal component casting or through wear, fatigue, or creep during service, and can require the component to be scrapped or repaired. The repair of single crystal superalloys is problematic because of the tendency for stray grains to form during welding and for solidification cracks to occur. 
         [0003]    Casting of single crystal (SX) alloys requires crystallographic selection from a directionally solidified seed crystal using a helical single crystal selector and unidirectional heat outflow to effect unidirectional solidification and SX extension. Lateral heat conduction and associated grain formation is avoided during casting by using refractory insulation and induction coils. This approach for directionally solidified and single crystal casting is taught for example by Gell, M. et al., “The Development of Single Crystal Superalloy Turbine Blades”, Pratt and Whitney Aircraft Group, published by The Minerals, Metal &amp; Materials Society (TMS), 1980. Successful repair of SX components requires similar heat management. Avoidance of stray grain (SG) formation is essential during original casting as well as repair. A maximum temperature gradient (G) and minimum growth velocity (V) has been found effective in eliminating SG formation and attendant solidification cracking. 
         [0004]    T. D. Anderson and J. N. Dupont describe in “Stray Grain Formation and Solidification Cracking Susceptibility of Single Crystal Ni-Base Superalloy CMSX-4”, American Welding Society, Welding Journal, 2011, reducing SG formation and associated cracking by utilizing high energy density processes such as electron beam (EB) welding. They conclude: “. . . EB process produces a higher temperature gradient that leads to reduced stray grains and less solidification cracking.” 
         [0005]      FIG. 1  illustrates a relationship between a weld process travel speed and stray grain formation in electron beam welding and laser welding per Anderson et al. supra. Using high energy density at very low travel speeds, sufficient temperature gradient (G) and low growth rate (V) can be achieved such that low SG content results. Modest increases in travel speed have minimal affect on G but increase V, resulting in high SG content with a worst condition resulting at about 320 mm/min. Larger increases in travel speed produce larger G and, because G is a more dominant factor than V in affecting SG content, a progressively lower SG content results with further increases in travel speed. The highest travel speeds to minimize SG formation are of the order of 1500 mm/min. This exceeds most practical repair cladding. However, good results are also obtained at speeds on the order of 100 mm/min which is practical for repair processes. 
         [0006]    Electron beam processing can produce a lower SG content than other energy delivery technologies, but it is uneconomical because of the requirement for high vacuum. Repetitive sequencing of components into a chamber, aligning them, and evacuating the chamber before processing is slow and inherently expensive. Another issue with electron beam processing is that top surface cooling is not controllable. In a vacuum there is radiative heat transfer but essentially no convection. Cooling is largely dictated by substrate conduction. Anderson et al. supra note that the top central region of the weld is prone to stray grain formation due to low G/V from a low thermal gradient where the temperature is distributed across the thickest region of the melt. 
         [0007]      FIG. 2  illustrates heat outflow vectors  26 ,  27 ,  28 , including lateral outflow  27 , from a weld melt pool  20  on a surface  23  of a crystalline substrate  22  that has a single crystal preferred grain orientation  24 . In the absence of lateral insulative or heat generating mechanisms used in casting (Gell et al. supra) cooling is dependent on convection and radiation  26  and conduction  27 ,  28 . Even with a generally flat bead to maintain a constant G, lateral outflow promotes stray grains and lateral crystallization. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention is explained in the following description in view of the drawings that show: 
           [0009]      FIG. 1  illustrates a known relationship between stray grain formation and weld process travel speed for electron beam welding and laser beam welding. 
           [0010]      FIG. 2  is a sectional view of a prior art crystalline substrate with a welding melt pool undergoing both axial and non-axial heat outflow leading to stray grain formation. 
           [0011]      FIG. 3  is a sectional view of a melt pool covered by slag of varying thickness that controls heat outflow. 
           [0012]      FIG. 4  shows a melt pool for a second layer formed on top of a first layer. 
           [0013]      FIG. 5  shows an embodiment as in  FIG. 4  with insulative borders beside the slag. 
           [0014]      FIG. 6  shows an embodiment as in  FIG. 4  with insulative borders beside the melt pool. 
           [0015]      FIG. 7  shows a melt pool in a repair excavation with insulation on the substrate surface beside the excavation to limit lateral heat outflow. 
           [0016]      FIG. 8  shows a melt pool in a repair excavation with laser warming on the substrate beside of the excavation to limit lateral heat outflow. 
           [0017]      FIG. 9  shows the embodiment of  FIG. 8  after stopping the melting energy but continuing the substrate warming energy to limit lateral heat outflow. 
           [0018]      FIG. 10  shows a melt pool in a bevel of a crystalline alloy substrate and an insulative barrier limiting liquid flow on a lower side of the melt pool. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 3  shows a melt pool  20  with a convex free surface  21  on a crystalline substrate  22  that has a single crystal preferred grain orientation  24 . Herein “free surface” means the surface of the melt pool not in contact with the substrate. Flux is added to the melt pool for example by mixing the flux with a powdered alloy filler material, or by forming composite particles of filler metal and flux, or by adding a flux layer above and/or below the filler metal, or by feeding flux and filler metal together as powders and/or via feed wires or other means. The flux is constituted to form an insulative slag  30  with a predetermined viscosity at the melt pool temperature (i.e. at temperatures of the slag when the melt pool is liquid) that causes the flux to flow into a heat outflow control geometry with a first thickness T 1  normal to the melt pool surface over a center of the melt pool, a second normal thickness T 2  of at least twice the first thickness above all sides of the melt pool, and a third lateral thickness T 3  of at least 4 times the first thickness around all sides of the melt pool as measured adjacent to and parallel to the substrate surface  23 . In one embodiment, thickness T 1  is no greater than 0.5 mm. This geometry is created by constituting the flux for adequate fluidity on the melt pool. For example the flux may be constituted with certain proportions of CaF 2  and similar fluorides, including but not limited to Na 3 AlF 6 , K 2 ZrF 6 , NaF, BaF 2 , LiF, MgF 2 , and StF 2 . The slag geometry controls the heat outflow vectors  26 A-C differentially across the melt pool to maintain substantially uniaxial heat outflows from the melt pool. 
         [0020]      FIG. 4  shows a melt pool  20 B for a second layer formed on top of the first layer  20 A now solidified as a single crystal extension of the substrate  22  and with original slag layer  30  removed. Slag  30 B provides a blanket of insulation of varying thickness on the second melt pool to manage heat outflow. Laser energy can be modulated across the width W of the melt pool to effect melting of preplaced powder, and fusion to the underlying substrate, as well as formation of slag to the sides of the deposit thereby effecting lateral insulation. Surface tension of molten metal and of molten slag and physical containment by way of solidified slag all act to define the lateral geometry of the final solidified metal deposit. The surface tension of molten metal may be high enough, the slag viscosity may be high enough, the slag fluidity may be low enough and the slag solidification temperature may be low enough that the side areas T 3  build-up vertically as shown due to slag solidification from the substrate surface upward. 
         [0021]      FIG. 5  shows a melt pool  20 B for a second layer formed on top of the first layer  20 A now solidified as a single crystal extension of the substrate  22 . This embodiment is useful for slag with a viscosity too low, or fluidity too high, or solidification temperature too high to support the vertical side build-up areas T 3  of  FIG. 4 . Slag  30 B provides a blanket of insulation of varying thickness on the second melt pool to manage heat outflow. Refractory insulating elements  32 A-B may laterally border the slag  30 B to limit lateral flow of the slag and to further provide lateral heat insulation, either on all layers or only on the second and subsequent layers as needed. Such insulating elements may be made for example of alumina and/or zirconia foam with at least one closed-cell surface  34 . Alternately, sintered alumina and/or zirconia powder or loose powder may be used. Such insulative elements may also include integral heating elements  33  for additional energy management. 
         [0022]      FIG. 6  shows a melt pool  20 B for a second layer formed on top of the first layer  20 A now solidified as a single crystal extension of the substrate  22 . Slag  30 B provides a blanket of insulation of varying thickness on the second melt pool to manage heat outflow. Refractory insulating elements  32 A-B may surround the melt pools  20 A,  20 B to limit lateral flow of melt pools and slag for both layers, or such elements may be used only on the second and subsequent layers as needed. The insulating elements may be for example refractory foam blocks of alumina and/or zirconia, and may have at least one closed-cell surface  34 . Alternately, sintered alumina and/or zirconia powder or loose powder may be used. Such insulative elements may also include integral heating elements  33  for additional energy management. 
         [0023]      FIG. 7  shows a melt pool  20 C in a repair excavation  36  in a surface  23  of a crystalline alloy substrate  22  with a single crystal preferred grain orientation  24 . Slag  30 C on the melt pool may be constituted to be more thermally conductive and/or to have higher emissivity than the melt pool. Insulating elements  38 A,  38 B are disposed immediately beside and around the excavation to block radiation and convection therefrom. The insulating elements may be formed of an insulating powder such as zirconia, or refractory foam blocks of zirconia and/or alumina. In one embodiment a single powdered flux material may be used for both the lateral insulation  38 A-B and the central conductive/emissive slag  30 C where the flux powder is insulative in powder form and conductive/emissive when molten. The laser beam  40  may be directed to melt only the portion of the flux in the excavation. 
         [0024]      FIG. 8  shows a melt pool  20 C in a repair excavation  36  in a surface  23  of a crystalline alloy substrate  22  with a single crystal preferred grain orientation  24 . Slag  30 C on the melt pool may be constituted to be more thermally conductive and/or to have higher emissivity than the melt pool. Laser energy may be applied at a melting level  40 A to additive alloy material and flux in the excavation to form the melt pool  20 C, and at a lesser warming level  40 B to the substrate surface  23  beside the melt pool to suppress or slow lateral heat outflow from the melt pool. As shown in  FIG. 9 , the side energy  40 B may be applied after, or continued after, the melt energy  40 A is removed. Alternately, or in addition, the energy  40 B may be applied as preheat energy and before melt energy  40 A is introduced. The side energy may be gradually decreased as the melt pool cools to maintain zero lateral heat outflows over a crystal growth time. This provides uniaxial heat outflows  26 D at a rate controlled by the flux composition and thickness. 
         [0025]      FIG. 10  shows a melt pool  20 D in a bevel  42  of a crystalline alloy substrate  22  with a single crystal preferred grain orientation  24 . An insulative barrier, such as a powder, sintered powder, or refractory foam block of material  44  such as alumina and/or zirconia may border and contain the slag at the downhill side of the bevel. Slag  30 D on the melt pool is constituted to flow into a geometry with differing thickness as previously described. The need for such physical containment is a function of surface tension and gravitational forces acting to affect geometry of the liquids of melt pool and slag as well as the need to maintain heat flow parallel to grain orientation  24 . Laser energy may be applied at a melting level  40 A to additive alloy material or to the substrate surface  23  to form the melt pool  20 D. A lesser warming level of laser energy  40 B may be applied to the substrate beside the melt pool to minimize lateral heat outflow from the melt pool. Additionally, a lesser warming level of laser energy and/or supplemental heating may be applied to insulative material  44  to minimize lateral heat outflow. The side energy  40 B may be applied before, after, or continued after the melt energy  40 A is removed. The side energy may be gradually decreased as the melt pool cools to maintain zero lateral heat flow. 
         [0026]    Materials for the above described fluxes may be divided into those providing insulative slag and those providing conductive and/or radiative slag. The insulative, conductive and spectrally emissive properties of molten slag are specifically important in this context because shortly after the slag solidifies the underlying metal solidifies and grain orientation is thereafter fixed. The thermal conductivity of molten slags has been reported to increase with increasing silica (SiO 2 ) content. (Ref. Mills, K., The Estimation of Slag Properties, Dept. of Materials—Imperial College, UK, March 2011.) The effect is related to slag structure and involves phonon conduction. So, a flux with high silica content is useful in conductive molten slag embodiments. Attempts to study conductivity of molten slags of other composition have been experimentally difficult. Higher amounts of CaF 2  have been reported to have higher thermal conductivity—relative to combination with CaO but the data is limited. (Ref. Commission of the European Communities, Physical Properties of Slags EUR 7292EN, 1981.) So, a flux with relatively high CaF 2  content and low CaO content may also be useful in conductive molten slag embodiments. The value of emissivity of CaF 2  in the liquid phase is about 0.97. The value of emissivity is only slightly lowered with the addition of Al 2 O 3  but is significantly reduced with the addition of MgO. (Ref. Commission of the European Communities, Physical Properties of Slags EUR 7292EN, 1981.) So, fluxes with high CaF 2 , high Al 2 O 3  and low MgO contents are useful in conductive and radiative slag embodiments. 
         [0027]    An exemplary flux for a molten conductive/radiative slag may comprise:
       10-60 wt. % CaF 2  for fluidity, thermal conductivity and emissivity;   10-60 wt. % SiO 2  for thermal conductivity;   10-60 wt. % Al 2 O 3  for emissivity;   less than 10 wt. % MgO to preserve emissivity; and   less than 10 wt. % CaO to preserve conductivity.       
 
         [0033]    An exemplary flux for an insulative molten slag may comprise:
       10-60 wt. % total of at least one of CaF 2  CaO, and MnO as fluidizers to enhance slag distribution and thickening at deposit edges to improve lateral insulation;   10-60 wt. % total of at least one of ZrO 2  and CaO as insulative constituents;   less than 20 wt. % SiO 2  to minimize negative effects on insulation and fluidity;   less than 30 wt. % Al 2 O 3  for slag structural building without excessively increasing thermal conductivity relative to ZrO 2  and CaO; and   less than 10 wt. % MgO to avoid excess thermal conductivity.       
 
         [0039]    Some flux constituents can dissociate in the hotter regions of processing and can reform or form new compounds in the cooler regions of processing. Such reformations can be exothermic. To the extent that they concentrate at the edges of a deposit, such heat release can effectively limit lateral heat outflow. This can be an alternate or addition to the lateral laser warming  40 B described previously. CaO can react with water vapor adjacent or above the deposit, forming Ca(OH) 2  and releasing heat. Thus in one embodiment it is beneficial to include up to 15 wt. % of CaO in the flux composition. 
         [0040]    This invention solves the challenge of avoiding stray grain formation and maintaining crystallographic orientation during repair of single crystal alloys. The specialized flux compositions and associated heat control methods herein produce successful laser repairs for single crystal alloys. It is beneficial to use flux as taught herein instead of inert gas for deposition of single crystal alloys, because the flux can control the heat outflow vectors differentially across the melt pool to provide substantially uniaxial heat outflow. 
         [0041]    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.