Abstract:
A process for welding directionally solidified metallic materials is presented. Process parameters are targeted selected with respect to laser welding, advancement, laser power beam diameter and powder mass flow. The temperature gradient, which is fundamentally decisive for the single-crystal growth during laser cladding, may be set in a targeted manner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2012/056739 filed Apr. 13, 2012 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the European application No. 11165301.0 EP filed May 09, 2011, the entire contents of which is hereby incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a process for welding directionally solidified metallic materials. 
       BACKGROUND OF INVENTION 
       [0003]    SX nickel-based superalloys reinforced with γ′ cannot be subjected to build-up welding with fillers of the same type in overlapping welding tracks in one or more layers either by means of conventional welding processes or by high-energy processes (laser, electron beam). The problem is that a microstructure with misorientation already forms in the case of an individual welding track in the marginal region close to the surface. For the subsequent overlapping track, this means that the solidification front in this region has no available SX nucleus, and the region with misorientation (no SX microstructure) expands further in the overlapping region. Cracks are formed in this region. 
         [0004]    For SX nickel-based superalloys reinforced with γ′, the welding processes used to date are not able to homogeneously build up a weld metal by overlapping in one or more layers with an identical SX microstructure. In the case of a single track on an SX substrate, the local solidification conditions vary in such a manner that, depending on the position, dendritic growth is initiated proceeding from the primary roots or the secondary arms. In this case, of the various possible dendrite growth directions, the direction which prevails is the direction with the most favorable growth conditions, i.e. the direction with the smallest angle of inclination with respect to the temperature gradient. The cause of the formation of misorientations in the SX microstructure during the powder build-up welding of SX nickel-based superalloys reinforced with γ′ has not yet been completely clarified. It is suspected that, when the dendrites meet one another from various growth directions, secondary arms may break away and serve as nuclei for the formation of a misoriented microstructure. In addition, powder particles which have not completely melted in the melt may serve as nuclei for the formation of a misoriented microstructure in the marginal region close to the surface. To solve this problem, a procedure which involves realizing growth conditions which favor only one growth direction for the dendrites is therefore proposed for the powder build-up welding of SX nickel-based superalloys reinforced with γ′. In addition, the procedure ensures that the powder particles are melted completely in the melt. 
       SUMMARY OF INVENTION 
       [0005]    Therefore, it is an object of the invention to solve the problem mentioned above. 
         [0006]    This object is achieved by a process as claimed in the independent claim. 
         [0007]    To solve this technical problem relating to the formation of a non-single-crystal microstructure in the marginal region of a single track close to the surface, a procedure is proposed for build-up welding with laser radiation in which this problem does not arise or arises to such a small extent that overlapping in one or more layers is possible without the formation of cracks at room temperature. 
         [0008]    The dependent claims list further advantageous measures which can be combined with one another, as desired, in order to obtain further advantages. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  shows a schematic course of the process, 
           [0010]      FIG. 2  shows a gas turbine, 
           [0011]      FIG. 3  shows a turbine blade or vane, 
           [0012]      FIG. 4  shows a list of superalloys, 
           [0013]      FIGS. 5 and 6  show welding beads. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0014]    The description and the figures represent only exemplary embodiments of the invention. 
         [0015]      FIG. 1  schematically shows the course of the process, with an apparatus  1 . 
         [0016]    The component  120 ,  130  to be repaired has a substrate  4  made of a superalloy, in particular of a nickel-based superalloy as shown in  FIG. 4 . Very particularly, the substrate  4  consists of a nickel-based superalloy. The substrate  4  is repaired by applying new material  7 , in particular by means of powder, to the surface  5  of the substrate  4  by build-up welding. 
         [0017]    This is effected by supplying material  7  and a welding beam, preferably a laser beam  10  of a laser, which melts at least the supplied material  7  and preferably also parts of the substrate  4 . Here, use is preferably made of powder. The diameter of the powder particles  7  is preferably so small that they can be melted completely by a laser beam and a sufficiently high temperature of the particles  7  results. In this respect, a melted region  16  and an adjoining solidification front  19  and, upstream thereof, an already resolidified region  13  are present on the substrate  4  during the welding. 
         [0018]    The apparatus of the invention preferably comprises a laser (not shown) with a powder supply unit and a movement system (not shown), with which the laser beam interaction zone and the impingement region for the powder  7  on the substrate surface  5  can be moved. In this case, it is preferable that the component (substrate  4 ) is neither preheated nor overaged by means of heat treatment. That region on the substrate  4  which is to be reconstructed is preferably subjected to build-up welding in layers. The layers are preferably applied in a meandering manner, unidirectionally or bidirectionally, in which case the scan vectors of the meandering movements from layer to layer are preferably turned in each case by 90°, in order to avoid bonding errors between the layers. The dendrites  31  in the substrate  4  and the dendrites  34  in the applied region  13  are shown in  FIG. 1 . 
         [0019]    A system of coordinates  25  is likewise shown. The substrate  4  moves relatively in the x direction  22  at the scanning speed V V . The z temperature gradient 
         [0000]    
       
         
           
             
               ∂ 
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               Z 
             
           
         
       
     
         [0000]      28  is present on the solidification front  19 . 
         [0020]    The welding process is carried out with process parameters concerning feed rate V v , laser power, beam diameter and powder mass flow which lead to a local orientation of the temperature gradient on the solidification front which is preferably smaller than 45° with respect to the direction of the dendrites  31  in the substrate  4 . This ensures that exclusively that growth direction which continues the dendrite direction  32  in the substrate  4  is favored for the dendrites  34 . This requires a beam radius which ensures that that part of the three-phase lines which delimits the solidification front  19  is covered completely by the laser beam. 
         [0021]    The approximative condition for a suitable inclination of the solidification front  19  with respect to the dendrite direction  32  of the dendrites  31  in the substrate  4  is preferably the following: 
         [0000]    
       
         
           
             
               
                 
                   
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         A: Degree of absorption of the substrate, 
         I L : Laser intensity, 
         V V : Scanning speed, 
         λ: Thermal conductivity of the substrate, 
         T: Temperature. 
       
     
         [0027]    The condition gives rise to a process window, depending on the material, concerning the intensity of the laser radiation (approximate top hat), the beam radius relative to the powder jet focus, the feed rate V V  and the powder mass flow. 
         [0028]    The complete coverage of the melt with the laser radiation ensures, in the case of the coaxial procedure, a longer time of interaction between the powder particles and the laser radiation and a consequently higher particle temperature upon contact with the melt. 
         [0029]    The particle diameter and therefore the predefined time of interaction should bring about a temperature level which is high enough for complete melting. Given an appropriate particle temperature and residence time in the melt, a sufficiently high temperature level of the melt should have the effect that the particles melt completely. 
         [0030]    By virtue of the process parameters and mechanisms described above, the prerequisites for epitaxial single-crystal growth in the weld metal with an identical dendrite orientation in the substrate are ensured. Since only one dendrite growth direction normal to the surface is activated during the welding process, the subsequent flowing of the melt into the interdendritic space is facilitated during solidification, and the formation of hot cracks is avoided. This results in a weld quality which is acceptable for structural welding (e.g. for the purposes of repairing or joining in a region of the component subject to a high level of loading). 
         [0031]    The relative speed V V  is preferably between 30 mm/min and 100 mm/min, and is preferably 50 mm/min. The power is in the range of preferably 200 W to 500 W, and is very preferably 300 W, the laser beam on the surface having a diameter of 3 mm to 6 mm, preferably 4 mm. The mass feed rate is preferably 300 mg/min to 600 mg/min, preferably 400 mg/min. 
         [0032]    In comparison to the prior art, the criterion G n /v is not used or is used only temporarily for a single-crystal or columnar dendritic solidification (see work of M. Gäumann). In comparison to the prior art, the developed process adapts the aforementioned process parameters of laser beam diameter, laser power, movement speed, powder mass flow in such a way that the track ( FIG. 5 ) or tracks ( FIG. 6 ) subjected to build-up welding solidify entirely in single-crystal form with a dendrite orientation (see  FIG. 5 ). This microstructure which is formed reduces the susceptibility to the formation of misorientated grains and therefore the formation of cracks by a continuous ductile interdendritic matrix also of the horizontal stresses during the build-up welding of multi-layered layers (see  FIG. 6 ). 
         [0033]      FIG. 2  shows, by way of example, a partial longitudinal section through a gas turbine  100 . In the interior, the gas turbine  100  has a rotor  103  with a shaft  101  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . The annular combustion chamber  110  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . 
         [0034]    Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row 125 are fitted to the rotor  103  for example by means of a turbine disk  133 . A generator (not shown) is coupled to the rotor  103 . 
         [0035]    While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
         [0036]    While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0037]    To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
         [0038]    By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0039]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). Alloys of this type are known from EP 0 486 489 B1,EP 0 786 017 B1,EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0040]    It is also possible for a thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0041]    The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 . 
         [0042]      FIG. 3  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0043]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  and a blade or vane tip  415 . As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0044]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof 
         [0045]    Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
         [0046]    Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
         [0047]    In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
         [0048]    Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0049]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 Al. The density is preferably 95% of the theoretical density. 
         [0050]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). The layer preferably has a composition Co—30Ni-28Cr-8A1-0.6Y-0.7Si or Co-28Ni-24Cr-10A1-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni—10Cr—12Al—0.6Y—3Re or Ni—12Co—21Cr—11Al—0.4Y—2Re or Ni—25Co—17Cr—10Al—0.4Y—1.5Re. 
         [0051]    It is also possible for a thermal barrier coating, which is preferably the outermost layer, to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. The thermal barrier coating covers the entire MCrAlX layer. 
         [0052]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0053]    Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0054]    Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused. 
         [0055]    The blade or vane  120 ,  130  may be hollow or solid in form. If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines).