Abstract:
Single-crystalline or stem-shaped turbine blades are difficult to produce. A turbine blade is provided which has varying structures for different areas of the turbine blade, wherein the airfoil region always includes a stem-shaped or single crystalline structure and the other regions may deviate therefrom. A method for producing the turbine blade is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2010/050121, filed Jan. 8, 2010 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 09000800 EP filed Jan. 21, 2009. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a component having a structure which differs in different places, and to a method for production. 
       BACKGROUND OF INVENTION 
       [0003]    In the single-crystal (SX) or rod-crystalline (DS) directional solidification of gas turbine blades, defect-free solidification of the blade root represents a challenge owing to the limited thermal gradients and the complex geometry in the precision casting furnace. 
         [0004]    In order to let the blade root solidify directionally (SX, DS), a very long and cost-intensive process cycle is required. Nevertheless, a not inconsiderable number of blades fail owing to grain structure defects (for example new grains) in the root. Furthermore, SX or DS solidification is limited in respect of the blade size, so that the advantages of directionally solidified turbine blades cannot be exploited in the rows of last rotor blades. 
         [0005]    According to the current prior art, directional solidification is carried out by the so-called Bridgeman method. In this case, blades are directionally solidified in a rod-crystalline (DS) or single-crystal (SX) fashion up to a size limited by the thermal gradients in the casting furnace. Defect-free solidification of the blade root, however, requires a very long process cycle. 
         [0006]    In order to overcome these limitations, on the plant side attempts are being undertaken to improve the thermal gradients acting in the furnace (for example by gas cooling, cooling in a ceramic fluidized bed or in a liquid metal melt (LMC)). Alloys have furthermore been optimized in respect of their castability, although this has usually entailed a compromise in the mechanical properties. 
       SUMMARY OF INVENTION 
       [0007]    It is an object of the invention to overcome the problems mentioned above. 
         [0008]    The object is achieved by a component as claimed in the claims, namely by columnar (DS) or conventional (CC) solidification in a second region with single-crystal (SX) solidification of a first region, and methods as claimed in the claims. 
         [0009]    Further advantageous measures, which may be combined with one another in any desired way in order to achieve further advantages, are listed in the dependent claims. 
         [0010]    It has been established that an SX or DS structure is not required in the second region owing to a lower loading temperature during operation and a differently acting loading profile. To this end, however, the alloy may require chemical elements which an alloy intended for SX/DS solidification does not necessarily contain. 
         [0011]    To this end, the alloy is advantageously to be modified in the second region (blade root), or an alloy which can accommodate the loading both in the second region (root) and the first region (blade surface) is to be used for the entire component. 
         [0012]    The entire solidification of such a blade may be carried out in situ in a process according to the three following technical features: 
         [0013]    1. DS-SX solidification of the blade surface (1 st  region) with a reduced amount of starting material (1 st  alloy for the blade surface, i.e. the amount for a melt that only provides the blade surface, but not the blade root); 
         [0014]    then switching over the process parameters and adding a second alloy (different to the 1 st  alloy) in order to solidify the blade root (2 nd  region). 
         [0015]    2. DS-SX solidification of the blade surface (1 st  region) with the full amount of starting material (alloy for the blade surface, blade platform and blade root, amount for a melt sufficient for the blade surface and blade root); 
         [0016]    then switching over the process parameters and adding additional alloy elements to the alloy of the blade surface, i.e. the not yet solidified melt of the full amount of starting material; 
         [0017]    solidifying the blade root (2 nd  region). 
         [0018]    3. DS-SX solidification of the blade surface (1 st  region) with the full amount of starting material (a single alloy is suitable both for the blade surface and for the root and in a sufficient amount); 
         [0019]    then switching over the process parameters and solidifying the blade root (2 nd  region). 
         [0020]    The amount of starting material is the amount of material of an alloy, or two alloys, which is necessary in order to completely cast the entire component or blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIGS. 1 to 8  show exemplary embodiments of a component, 
           [0022]      FIG. 9  shows a gas turbine, 
           [0023]      FIG. 10  shows a turbine blade, 
           [0024]      FIG. 11  shows a list of superalloys. 
       
    
    
       [0025]    The description and the figures merely represent exemplary embodiments of the invention. 
       DETAILED DESCRIPTION OF INVENTION 
       [0026]    The invention will be described merely by way of example with reference to a turbine blade  120 ,  130 . 
         [0027]      FIG. 1  represents a turbine blade  120  comprising a blade surface region  406  (first region  406 ), a blade platform  403  (first  406  and/or second region  403 ) and a fastening region  400  (second region). 
         [0028]    The blade surface region  406  preferably consists of a single-crystal structure (SX). The single-crystal structure (SX) extends from the blade tip  415  and preferably as far as the upper side  4  of the blade platform  403 . 
         [0029]    The blade platform  403  and at least the fastening region  400  preferably have a different structure, i.e. not a single-crystal structure. This may comprise: rod-shaped crystals solidified in columnar fashion (DS) or a nondirectional structure (CC structure). 
         [0030]    Depending on the mechanical requirement, the single-crystal structure (SX) of the blade surface  406  may also extend as far as a certain thickness into the blade platform  403 . In this case, a DS or CC structure begins inside the blade platform  403  ( FIG. 2 ). 
         [0031]    For particularly high loads (thermal, mechanical), the entire blade platform  403  may also be solidified in single-crystal fashion, so that only the fastening region  400  has a CC or DS structure, as represented in  FIG. 3 . 
         [0032]      FIG. 4  shows another exemplary embodiment of the invention,  FIG. 4  representing a similar example to  FIG. 1 , namely that the SX structure is replaced in the blade surface region  406  by a DS structure and the subsequent regions have a structure comprising a CC structure. This also applies similarly for a DS-CC structure according to  FIGS. 2 and 3 . 
         [0033]    The blade surface  406  may likewise have an SX structure, the blade platform  403  a DS structure and the blade root  400  a CC structure ( FIG. 5 ). 
         [0034]    If three structures (SX, DS, CC) are present, they may extend over different regions: 
         [0035]      FIG. 6 : SX in the blade surface  406 ,
       SX partially in the blade platform  403 ,   the remainder of the blade platform  403  is DS,   blade root  400 =CC         
         [0039]      FIG. 7 : SX in the blade surface  406 ,
       DS in the blade platform  403     and DS only partially in the blade root  400 ,   blade root  400 =CC         
         [0043]      FIG. 8 : SX in the blade surface  406         and SX only partially in the blade platform  403 ,   DS in the blade platform  403  and   DS only partially in the blade root  400 ,   CC in the blade root (remainder).         
         [0048]    When there is SX in the blade surface  406 , the blade platform  403  may likewise have DS and CC structures (as seen in the direction of the blade root  400 ) or SX, DS, CC structures (as seen in the direction of the blade root  400 ), the blade root respectively having a CC structure. 
         [0049]    The advantages of the different structures are: 
         [0050]    reducing the reject rate in the production of SX or DS components 
         [0051]    significant cost reduction in the process management 
         [0052]    utilization of SX-DS structures for larger blades and, associated with this, a possible increase in the turbine efficiency, 
         [0053]    local optimization of the blade root or blade surface in respect of the locally acting loading profile. 
         [0054]      FIG. 9  shows a gas turbine  100  by way of example in a partial longitudinal section. 
         [0055]    The gas turbine  100  internally comprises a rotor  103 , which will also be referred to as the turbine rotor, mounted so as to rotate about a rotation axis  102  and having a shaft  101 . 
         [0056]    Successively along the rotor  103 , there are an intake manifold  104 , a compressor  105 , an e.g. toroidal combustion chamber  110 , in particular a ring combustion chamber, having a plurality of burners  107  arranged coaxially, a turbine  108  and the exhaust manifold  109 . 
         [0057]    The ring combustion chamber  110  communicates with an e.g. annular hot gas channel  111 . There, for example, four successively connected turbine stages  112  form the turbine  108 . 
         [0058]    Each turbine stage  112  is formed for example by two blade rings. As seen in the flow direction of a working medium  113 , a guide vane row  115  is followed in the hot gas channel  111  by a row  125  formed by rotor blades  120 . 
         [0059]    The guide vanes  130  are fastened on an inner housing  138  of a stator  143  while the rotor blades  120  of a row  125  are fitted on the rotor  103 , for example by means of a turbine disk  133 . 
         [0060]    Coupled to the rotor  103 , there is a generator or a work engine (not shown). 
         [0061]    During operation of the gas turbine  100 , air  135  is taken in and compressed by the compressor  105  through the intake manifold  104 . The compressed air provided at the turbine-side end of the compressor  105  is delivered to the burners  107  and mixed there with a fuel. The mixture is then burnt to form the working medium  113  in the combustion chamber  110 . From there, the working medium  113  flows along the hot gas channel  111  past the guide vanes  130  and the rotor blades  120 . At the rotor blades  120 , the working medium  113  expands by imparting momentum, so that the rotor blades  120  drive the rotor  103  and this drives the work engine coupled to it. 
         [0062]    The components exposed to the hot working medium  113  experience thermal loads during operation of the gas turbine  100 . Apart from the heat shield elements lining the ring combustion chamber  110 , the guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the flow direction of the working medium  113 , are heated the most. 
         [0063]    In order to withstand the temperatures prevailing there, they may be cooled by means of a coolant. 
         [0064]    Substrates of the components may likewise comprise a directional structure, i.e. they are single-crystal (SX structure) or comprise only longitudinally directed grains (DS structure). 
         [0065]    Iron-, nickel- or cobalt-based superalloys are for example used as the material for the components, in particular for the turbine blades  120 ,  130  and components of the combustion chamber  110  ( FIG. 11 ). 
         [0066]    Such superalloys 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. 
         [0067]    The blades or vanes  120 ,  130  may also have coatings against corrosion (MCrAlX: M is at least one element from the group 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). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0068]    On the MCrAlX, there may furthermore be a thermal barrier layer, which consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0069]    Rod-shaped grains are produced in the thermal barrier layer by suitable coating methods, for example electron beam deposition (EB-PVD). 
         [0070]    The guide vane  130  comprises a guide vane root (not shown here) facing the inner housing  138  of the turbine  108 , and a guide vane head lying opposite the guide vane root. The guide vane head faces the rotor  103  and is fixed on a fastening ring  140  of the stator  143 . 
         [0071]      FIG. 10  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0072]    The turbomachine may be a gas turbine of an aircraft or of a power plant for electricity generation, a steam turbine or a compressor. 
         [0073]    The blade  120 ,  130  comprises, successively along the longitudinal axis  121 , a fastening region  400 , a blade platform  403  adjacent thereto as well as a blade surface  406  and a blade tip  415 . 
         [0074]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0075]    A blade root  183  which is used to fasten the rotor blades  120 ,  130  on a shaft or a disk (not shown) is formed in the fastening region  400 . 
         [0076]    The blade root  183  is configured, for example, as a hammerhead. Other configurations as a firtree or dovetail root are possible. 
         [0077]    The blade  120 ,  130  comprises a leading edge  409  and a trailing edge  412  for a medium which flows past the blade surface  406 . 
         [0078]    In conventional blades  120 ,  130 , for example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade  120 ,  130 . 
         [0079]    Such superalloys 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. 
         [0080]    The blade  120 ,  130  may in this case be manufactured by a casting method, also by means of directional solidification, by a forging method, by a machining method or combinations thereof. 
         [0081]    Workpieces with a single-crystal structure or single-crystal structures are used as components for machines which are exposed to heavy mechanical, thermal and/or chemical loads during operation. 
         [0082]    Such single-crystal workpieces are manufactured, for example, by directional solidification from the melts. These are casting methods in which the liquid metal alloy is solidified to form a single-crystal structure, i.e. to form the single-crystal workpiece, or is directionally solidified. 
         [0083]    Dendritic crystals are in this case aligned along the heat flux and form either a rod crystalline grain structure (columnar, i.e. grains which extend over the entire length of the workpiece and in this case, according to general terminology usage, are referred to as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of a single crystal. It is necessary to avoid the transition to globulitic (polycrystalline) solidification in these methods, since nondirectional growth will necessarily form transverse and longitudinal grain boundaries which negate the beneficial properties of the directionally solidified or single-crystal component. 
         [0084]    When directionally solidified structures are referred to in general, this is intended to mean both single crystals which have no grain boundaries or at most small-angle grain boundaries, and also rod crystal structures which, although they do have grain boundaries extending in the longitudinal direction, do not have any transverse grain boundaries. These latter crystalline structures are also referred to as directionally solidified structures. 
         [0085]    Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0086]    The blades  120 ,  130  may also have coatings against corrosion or oxidation, for example MCrAlX (M is at least one element from the group 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)). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0087]    The density is preferably 95% of the theoretical density. 
         [0088]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an interlayer or as the outermost layer). 
         [0089]    The layer composition preferably comprises Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. Besides 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. 
         [0090]    On the MCrAlX, there may furthermore be a thermal barrier layer, which is preferably the outermost layer and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0091]    The thermal barrier layer covers the entire MCrAlX layer. 
         [0092]    Rod-shaped grains are produced in the thermal barrier layer by suitable coating methods, for example electron beam deposition (EB-PVD). 
         [0093]    Other coating methods may be envisaged, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier layer may comprise porous, micro- or macro-cracked grains for better thermal shock resistance. The thermal barrier layer is thus preferably more porous than the MCrAlX layer. 
         [0094]    Refurbishment means that components  120 ,  130  may need to be stripped of protective layers (for example by sandblasting) after their use. The corrosion and/or oxidation layers or products are then removed. Optionally, cracks in the component  120 ,  130  are also repaired. The component  120 ,  130  is then recoated and the component  120 ,  130  is used again. 
         [0095]    The blade  120 ,  130  may be designed to be a hollow or solid. If the blade  120 ,  130  is intended to be cooled, it will be hollow and optionally also comprise film cooling holes  418  (indicated by dashes).