Abstract:
The invention relates to a method of welding locally a surface of a Ni base, especially a single crystal superalloy substrate using a laser beam while preheating the substrate to an optimized temperature for the purpose of repairing cracks. Welding repair of single crystal super alloys often leads to two main types of defects: cracks and spurious grains. Both defects can be avoided using an optimized preheating temperature set to higher than 500° C.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European application No. 07019669.6 filed Oct. 8, 2007, which is incorporated by reference herein in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of welding locally the surface of a Ni base, especially a single crystal (SX) superalloy substrate using a laser beam while preheating the substrate to an optimized temperature for the purpose of repairing cracks. This is useful because blades are expensive, especially for single crystalline components (SX) components. 
       BACKGROUND OF THE INVENTION 
       [0003]    The U.S. Pat. No. 5,374,319 teaches that the preheating temperature during welding is 760° C., preferably at a higher temperature of 920° C. 
         [0004]    After casting or after service high temperature turbine parts (e.g. turbine blades or vanes) may present surface cracks that must be repaired prior processing. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore the aim of the invention to overcome this problem. 
         [0006]    The problem is solved by a method according the claims. Further advantageous steps are listed in the dependent claims which can be combined arbitrarily which each other to yield further advantages. 
         [0007]    A laser assisted process is foreseen for the repair of cracks affecting SX turbine parts by surface local controlled laser welding or remelting. 
         [0008]    When SX components are treated by a laser, two main types of defects might affect the repaired zone: spurious grains and solidification cracking. 
         [0009]    The conditions for successful SX repair on SX components require epitaxial and columnar growth and avoiding equiaxed or misoriented columnar growth responsible for grain boundaries formation. To guarantee a SX structure, a precise process control that insures epitaxial columnar growth is essential. 
         [0010]    Apart from the microstructure control, conditions which produce crack free solidification constitute a prerequisite for the repair of real parts. 
         [0011]    The rising of the temperature of the surrounding material through preheating constitute the most effective way to reduce the cooling rate and the cracking tendency. The preheating treatment generally used for gamma prime precipitation strengthened nickel base superalloys consists in heating the entire weld area to a ductile temperature set above the aging temperature (˜870° C.) and below the incipient melting temperature but might be defined as being set in between 950° C. and 1000° C. U.S. Pat. No. 5,374,319. 
         [0012]    Within this temperature range the thermal gradients are reduced by one or to order of magnitude and thus increase the risk for nucleation of spurious grains by increasing the driving force for nucleation. The process window for SX solidification is thus critically reduced which drastically limit the use of the SX laser assisted repair. 
         [0013]    Such high preheating temperatures also constitute a risk for the process upscale to real parts as it can trigger recrystallization of location presenting high dislocation density (e.g. blade roots). 
         [0014]    The limitation inherent to the use of the preheating treatment defined in the state of the art is solved trough the definition of a preheating treatment balancing those two conflicting features (spurious grain nucleation and hot cracking). 
         [0015]    The optimal preheating temperature here proposed is above 500° C. This particular temperature allows reducing the yield strength of the surrounding material and thus the associated restraint which usually restrict the required shrinkage of the weld bead and lead to tensile stress build-up in the critical area while holding the driving force for spurious grain nucleation to a sufficiently low value. 
         [0016]    The heating source employed may consist in an induction system allowing local heat treatment. 
         [0017]    Taking into account the somewhat low temperature here defined the use of infrared lamp or defocused laser beam might be conceivable to achieve the desired preheating temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention is further explained with reference to the following drawings. 
           [0019]      FIG. 1  shows a gas turbine, 
           [0020]      FIG. 2  shows a turbine blade, 
           [0021]      FIG. 3  shows a combustion chamber, 
           [0022]      FIG. 4 ,  5 ,  6  shows a component to be repaired by welding, 
           [0023]      FIG. 7  shows a listing of superalloys and 
           [0024]      FIG. 8 ,  9  experimental results. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0026]    In the interior, the gas turbine  100  has a rotor  103  which is mounted such that it can rotate about an axis of rotation  102 , has a shaft  101  and is also referred to as the turbine rotor. 
         [0027]    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 . 
         [0028]    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 . 
         [0029]    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 . 
         [0030]    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 . 
         [0031]    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. 
         [0032]    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 bricks which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0033]    To be able to withstand the temperatures which prevail there, they can be cooled by means of a coolant. 
         [0034]    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). 
         [0035]    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 . 
         [0036]    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. 
         [0037]    The guide vane  130  has a guide vane root (not shown here) facing the inner housing  138  of the turbine  108  and a guide vane head 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 . 
         [0038]      FIG. 2  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0039]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0040]    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  as well as a blade or vane tip  415 . 
         [0041]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0042]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or disk (not shown), is formed in the securing region  400 . 
         [0043]    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. 
         [0044]    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 . 
         [0045]    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 . 
         [0046]    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. 
         [0047]    The blade or vane  120 ,  130  may in this case be produced by a casting process, also by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0048]    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. 
         [0049]    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. 
         [0050]    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. 
         [0051]    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). 
         [0052]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0053]    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 represents yttrium (Y) and/or silicon and/or at least one rare earth element, or haffium (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 A1. 
         [0054]    The density is preferably 95% of the theoretical density. A protective aluminum oxide layer (TGO=thermally grown oxide layer) forms on the MCrAlX layer (as an intermediate layer or an outermost layer). 
         [0055]    It is also possible for a thermal barrier coating, 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, which is preferably the outermost layer, to be present on the MCrAlX. 
         [0056]    The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by means of suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0057]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include porous grains which have microcracks or macrocracks for improving its resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0058]    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). 
         [0059]      FIG. 3  shows a combustion chamber  110  of the gas turbine  100 . The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107  arranged circumferentially around an axis of rotation  102  open out into a common combustion chamber space  154  and generate flames  156 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0060]    To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
         [0061]    A cooling system may also be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then, for example, hollow and if appropriate also have cooling holes (not shown) opening out into the combustion chamber space  154 . 
         [0062]    Each heat shield element  155  made from an alloy is provided on the working medium side with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from high-temperature-resistant material (solid ceramic bricks). 
         [0063]    These protective layers may be similar to those used for the turbine blades or vanes, i.e. for example meaning 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 represents 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 A1. 
         [0064]    It is also possible for a, for example, ceramic thermal barrier coating, 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, to be present on the MCrAlX. 
         [0065]    Columnar grains are produced in the thermal barrier coating by means of suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0066]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have porous grains which have microcracks or macrocracks to improve its resistance to thermal shocks. 
         [0067]    Refurbishment means that after they have been used, protective layers may have to be removed from turbine blades or vanes  120 ,  130 , heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the turbine blade or vane  120 ,  130  or the heat shield element  155  are also repaired. This is followed by recoating of the turbine blades or vanes  120 ,  130 , heat shield elements  155 , after which the turbine blades or vanes  120 ,  130  or the heat shield elements  155  can be reused. 
         [0068]      FIG. 4  shows a component  1 ,  120 ,  130 ,  155 , which comprises a substrate  4 . 
         [0069]    The substrate  4  is especially made of a superalloy, especially of w nickel based superalloy. 
         [0070]    Superalloys which can be repaired by this method are listed in  FIG. 7 , especially: PWA1483SX, CMSX4. 
         [0071]    This substrate  4  posesses a crack  10  or hole  10  which has to be closed. The hole or crack  10  is a blind hole. 
         [0072]    Especially the depth of the cracks  10  is between 0.75 mm up to 1.5 umm. Especially the depth of the cracks  10  is up to 1 mm, very especially in the range of 1 mm. The width of the crack  10  at the surface  22  of the component is preferably in the range between 10 μm to 100 μm. 
         [0073]    The preheating is preferably performed only locally around the area  10  to be welded and in the other region the temperature is much lower. 
         [0074]    Very good results have been obtained at temperature &gt;500° C., especially in a temperature range between 510° C. and 550° C., because temperatures ≦500° C. lead to an increase of defects like misorientation of grains, because the thermal gradient is to high, by which yielding rates of good welds are decreased or number of defects decreases ( FIG. 9 ). “&gt;500° C.” means that the temperature T with a given measuring tolerance ΔT (&gt;0) is higher than 500° C.: T preheat &gt;500° C.+ΔT. 
         [0075]    The preheating temperature is preferably maintained during the whole welding process. 
         [0076]    Although there are several possibilities of lasers  13  as welding device to be used it was found that a Nd-YAG or high power diode laser type is the best to be used. 
         [0077]    The diameter of the spot size of the laser beam is in the range of 2.5 mm to 5 mm, especially from 3 mm to 5 mm and very especially in the range of 4 mm. 
         [0078]    Surprisingly it was found that such a big diameter of the laser beam focus shows good results of repairing that small cracks (10 μm to 100 μm), wherein “small” relates to the crack width at the surface  22  of the substrate  4 . 
         [0079]    The power of the laser  13  P Laser [W] is preferably between 450 Watt to 950 Watt, especially 500 Watt to 900 Watt ( FIG. 8 ), so that laser intensities of 2.3 kW/cm 2  to 30 kW/cm 2 , especially 2.5 kW/cm 2  to 29 kW/cm 2  are reached. 
         [0080]    Preferably the relative movement of the laser beam and the substrate  4  to be repaired is &lt;1 mm/s, especially ≦0.9 mm/s and very especially of 50 mm/min. Preferably the relative movement is ≧0.4 mm/s, especially ≧0.6 mm/s 
         [0081]    Nevertheless, additional material  19  ( FIG. 6 ), especially: PWA 1483, CMSX4 based powders can be added by a material feeder  16  ( FIG. 6 , especially in form of powders) whose supplied material is melted also by the welding apparatus  13 .