Abstract:
A method for producing a rotor blade for a gas turbine, the blade having a fastening region and a platform, on which a blade ending in a blade tip is arranged. The method is designed to facilitate a particular high efficiency of the gas turbine with a particularly resource-saving manner of production. The method includes: production of a reinforcement; casting of a first part of the rotor blade about at least one portion of the reinforcement; and construction of a second part of the rotor blade by a 3D-printing method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2015/075384 filed Nov. 2, 2015, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP14196293 filed Dec. 4, 2014. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for producing a rotor blade for a gas turbine, the blade having a fastening region and a platform, on which a blade airfoil ending in a blade tip is arranged. It furthermore relates to a rotor blade of this kind. 
       BACKGROUND OF INVENTION 
       [0003]    Rotor blades of the abovementioned type are used in gas turbines to convert the energy of a hot gas flow into rotational energy. They typically have a blade airfoil, which is penetrated by cavities for carrying cooling air and which is fastened on a “platform”. Adjoining the platform there is a fastening region for fastening the turbine blade on the rotor or stator of the gas turbine. 
         [0004]    Currently, rotor blades for gas turbines are produced by casting from a single piece and a single material. This is generally performed by way of the vacuum precision casting method. As part of further improvement in the efficiency of current gas turbines, particularly by increasing the mass flow, it has been found, however, that stability problems can occur with turbine blades produced by this method. Moreover, the mold used to produce the cooling ducts is generally not reusable. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to specify a method and a rotor blade of the type stated at the outset which permit a particularly high efficiency of the gas turbine in combination with production which is particularly economical in terms of resources. 
         [0006]    As regards the method, this object is achieved, according to the invention, in that the method comprises the following method steps: —production of a reinforcement; —casting of a first part of the rotor blade around at least one part of the reinforcement; and —buildup of a second part of the rotor blade by means of a 3-D printing method. 
         [0007]    As regards the rotor blade, the object is achieved in that the rotor blade comprises a reinforcement arranged in the interior. 
         [0008]    Here, the invention starts from the consideration that the design of the turbine rotor blade stage, particularly of the fourth turbine rotor blade stage, comes up against limits in terms of production engineering when it comes to increasing the mass flow through the gas turbine: the rotor blade is supposed to have an internal structure described at the outset and to be actively cooled. In this case, determination and evaluation of critical parameters and characteristics derived from the flow area and rotational speed, particularly in the case of the fourth turbine rotor blade stage, have shown that it would be necessary to embody the component wall thickness in the upper region of the blade airfoil at about 1 mm within very narrow limits. In the case of such a large component, it is generally difficult to implement a wall thickness of 1 mm. This problem can be solved by not producing the rotor blades completely in the casting process but producing them by means of a 3-D printing method. In this process, very complex geometries can be produced in the interior of the blades. The problem is that lower component strength is achieved with the known 3-D printing methods. In order to increase the strength of the rotor blades produced by 3-D printing, the proposal is therefore to use a composite material. As with steel reinforced concrete, the intention is to insert stays of a stronger material into the base material. This reinforcement, together with said stays, is produced first of all, after which parts of the rotor blade are cast around the reinforcement and, finally, the finer parts of the rotor blades, in particular, e.g. the cooling air ducts, are built up by means of 3-D printing. 
         [0009]    Just as with steel reinforced concrete, the stays are intended to be aligned in the direction of the principal load. This applies especially to the load imposed by the centrifugal force during the operation of the gas turbine, which acts radially outward, i.e. in the direction of the blade tip, and increases continuously toward the blade tip. The reinforcement should therefore advantageously comprise at least one stay, which extends from a region of the rotor blade tip to the platform, and therefore through the entire blade airfoil in the direction of the load. Stabilization of the blade airfoil with respect to the occurring centrifugal force is thereby achieved. 
         [0010]    Stability is improved even further if the reinforcement advantageously comprises a stay which extends from the region of the rotor blade tip to the fastening region. The entire rotor blade structure, from the fastening region at the rotor blade root to the tip of the blade airfoil, is thereby stabilized by a continuous stay. 
         [0011]    As a further advantageous embodiment of the method, the above-described first part, which is cast around the reinforcement, comprises the fastening region. The fastening region, which generally has a tongue and groove structure resembling a firtree, generally comprises only a few cooling ducts, if any, and therefore has a less complex geometry. It is therefore also possible, in particular, to produce it using a reusable mold. Casting the fastening region furthermore ensures that said region has the required strength, and good cohesion with the reinforcement is produced. 
         [0012]    The same applies to the platform, which seals off the fastening region from the hot gas in the turbine duct. This platform too can be cast using a reusable mold and, as a result, has a particularly high strength. The cast first part therefore advantageously comprises the platform. 
         [0013]    As a further advantageous embodiment of the method, the cast first part of the rotor blade comprises a sheath for at least some of the stays of the reinforcement in one region of the blade airfoil. In other words: a sheath of solid material is cast around the reinforcement in the region of the blade airfoil. This sheath can be cast around the respective stay in a predetermined thickness, for example. A composite structure with the reinforcement, which acts as a substrate for 3-D printing, is thereby produced. 
         [0014]    It is advantageous if the reinforcement is manufactured from steel. In this context, currently used blade materials are suitable, in particular, e.g. high-strength steels or alloys with an appropriately high tensile strength and a high modulus of elasticity in comparison with the surrounding material. The high strength of such materials makes them particularly suitable for absorbing the centrifugal forces which occur during operation and for stabilizing the rotor blade. Moreover, they have a high melting point, and therefore they remain stable during the process of casting the blade root or the sheath. 
         [0015]    The second rotor blade part, which is built up by means of the 3-D printing method, advantageously comprises one region of the blade airfoil. It is precisely here that complex structures, e.g. cooling ducts, have to be produced, and these can be shaped in a particularly simple and flexible manner by means of 3-D printing. 
         [0016]    It is advantageous if these cooling ducts are arranged in the region of the blade airfoil during the buildup of the second part. 
         [0017]    In an advantageous embodiment of the method, selective laser fusion is used as the 3-D printing method. In selective laser fusion, the material to be processed is applied in powder form, in a thin layer, to the already cast part of the rotor blade. The powdered material is completely remelted locally by means of laser radiation and, after solidification, forms a solid layer of material. The rotor blade is then lowered by the amount of one layer thickness and powder is applied again. This cycle is repeated until all the layers have been remelted. 
         [0018]    It is advantageous if a rotor blade for a gas turbine is produced by one of the methods described up to this point. 
         [0019]    It is advantageous if a gas turbine comprises a rotor blade of this kind. 
         [0020]    The advantages achieved by means of the invention consist especially in that the weakness of 3-D printing materials is compensated by the use of a composite material and that such a method can be used for producing turbine blades. With the method, the wall thickness of the blade airfoil can be less, and it is also possible to produce longer rotor blades, thus improving the efficiency and power of the gas turbine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Illustrative embodiments of the invention are explained in greater detail by means of a drawing, in which: 
           [0022]      FIG. 1  show a partial longitudinal section through a gas turbine, 
           [0023]      FIG. 2  shows a reinforcement for a rotor blade, 
           [0024]      FIG. 3A  shows the reinforcement with a cast blade root in section, 
           [0025]      FIG. 3B  shows the reinforcement with a cast blade root and partial sheathing of the reinforcement, and 
           [0026]      FIG. 4  shows a section through the rotor blade. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0027]    In all the figures, identical parts are provided with the same reference signs. 
         [0028]      FIG. 1  shows a gas turbine  100  in a partial longitudinal section. A turbine is a continuous flow machine which converts the internal energy (enthalpy) of a flowing fluid (liquid or gas) into rotational energy and ultimately into mechanical driving energy. 
         [0029]    In the interior, the gas turbine  100  has a rotor  103 , which is mounted so as to rotate about an axis of rotation  102  (axial direction) and which is also referred to as a turbine rotor. Situated one after the other along the rotor  103  are an intake casing  104 , a compressor  105 , a toroidal combustion chamber  110 , in particular an annular combustion chamber  106 , having a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust casing  109 . 
         [0030]    The annular combustion chamber  106  communicates with an annular hot gas duct  111 . There, four turbine stages  112  arranged in series, for example, form the turbine  108 . Each turbine stage  112  is formed by two blade rings. When viewed in the flow direction of a working medium  113 , a guide blade row  115  is followed in the hot gas duct  111  by a row  125  formed by rotor blades  120 . The blades  120 ,  130  have a slightly curved profile, similar to an aircraft wing. 
         [0031]    In this case, the guide blades  130  are fastened on the stator  143 , whereas the rotor blades  120  of a row  125  are mounted on the rotor  103  by means of a turbine disk  133 . The rotor blades  120  thus form component parts of the rotor or turbine wheel  103 . A generator or a machine (not shown) is coupled to the rotor  103 . 
         [0032]    During the operation of the gas turbine  100 , air  135  is drawn in and compressed by the compressor  105  through the intake casing  104 . The compressed air supplied at the turbine end of the compressor  105  is passed to the burners  107  and is mixed there with a fuel. The mixture is then burnt in the combustion chamber  110  to form the working medium  113 . From the combustion chamber, the working medium  113  flows along the hot gas duct  111 , past the guide blades  130  and the rotor blades  120 . 
         [0033]    Some of the internal energy of the fluid flow is removed by the—as far as possible—eddy-free laminar flow of the turbine blades  120 ,  130 , and this energy is transferred to the rotor blades  120  of the turbine  108 . This then imparts rotation to the rotor  103 , as a result of which, first of all, the compressor  105  is driven. The useful power is output to the machine (not shown). 
         [0034]    During the operation of the gas turbine  100 , the components exposed to the hot working medium  113  are subject to thermal loads. Apart from the heat shield blocks lining the annular combustion chamber  106 , the guide blades  130  and rotor blades  120  of the first turbine stage  112 , as viewed in the flow direction of the working medium  113 , are subject to the highest thermal loads. The high loads render materials capable of bearing extreme loads necessary. The turbine blades  120 ,  130  are therefore manufactured from titanium alloys, nickel super alloy or tungsten-molybdenum alloys. The blades are protected by coatings against corrosion (MCrAlX; M=Fe, Co, Ni, rare earths) and heat (heat insulation layer, e.g. ZrO2, Y2O4-ZrO2) to ensure greater resistance to temperature and erosion, such as pitting. The heat shield coating is referred to as a thermal barrier coating or TBC for short. Further measures for making the blades more resistant to heat consist in sophisticated cooling duct systems. This technique is used both in the guide blades and in the rotor blades  120 ,  130 . 
         [0035]    Each guide blade  130  has a guide blade root (not shown here) facing the inner casing  138  of the turbine  108  and a guide blade head situated opposite the guide blade root. The guide blade head faces the rotor  103  and is fixed on a sealing ring  140  of the stator  143 . In this arrangement, each sealing ring  140  surrounds the shaft of the rotor  103 . Each rotor blade  120  likewise has a rotor blade root of this kind, as illustrated in the following figures, but ends in a rotor blade tip. 
         [0036]      FIG. 2  shows a reinforcement  144  for a rotor blade  120  of this kind. The reinforcement  144  has an upper transverse stay  146 . 
         [0037]    A total of four longitudinal stays  148 ,  150  point downward from the upper transverse stay  146  at equal spacings from one another, extending away from one another slightly in a radiating pattern. The two central longitudinal stays  148  are somewhat longer than the outer longitudinal stays  150 . They are of equal length and are connected at the end thereof by a lower transverse stay  152 . At the end of the outer longitudinal stays  150 , which are likewise of equal length, said stays are connected to one another and to the central longitudinal stays  150  by a further transverse stay  154 . Between the further transverse stay  154  and the upper transverse stay  146 , all the longitudinal stays  148 ,  150  are connected to one another by central transverse stays  156  arranged at regular spacings. 
         [0038]    The arrangement of the longitudinal and transverse stays  146 ,  148 ,  150 ,  152 ,  154 ,  156  which is shown in  FIG. 2  is only illustrative. In further alternative embodiments, it is, of course, possible to choose the number, orientation and distribution of the stays to match the forces which occur. 
         [0039]    The longitudinal and transverse stays  146 ,  148 ,  150 ,  152 ,  154 ,  156  thus form a stable reinforcement  144 , which can be arranged in the interior of a rotor blade  120 . The process for producing the rotor blade  120  is explained with reference to the following figures. First of all, the reinforcement  144  is produced as described. It consists of a high-strength steel alloy and remains stable in the casting process shown in  FIGS. 3A and 3B . 
         [0040]    In a first illustrative embodiment of the method, only the rotor blade root  158  is cast, as shown in  FIG. 3A . This root comprises a platform  160  for sealing off the inner regions of the rotor  103  from the hot gas in the hot gas duct  111  and comprises a fastening region  162 , which has a tongue on both sides, which is formed for a tongue and groove joint on the rotor. As can be seen, the transverse stay  154  is situated in the platform  160  here. Those regions of the central longitudinal stays  148  which extend downward beyond the transverse stay  154 , and the lower transverse stay  152 , are situated in the fastening region  162 . 
         [0041]    In a second illustrative embodiment of the method, sheaths  164  for each longitudinal stay  150 ,  152  are cast in addition to the rotor blade root  158 , as shown in  FIG. 3B . These sheaths can be of cylindrical design, for example, with the longitudinal stay  150 ,  152  as an axis. In both illustrative embodiments, a comparatively simple, reusable mold can be used in casting. 
         [0042]    Finally,  FIG. 4  shows the finished rotor blade  120 . The remaining parts of the body of the rotor blade  120  have been produced by means of selective laser fusion, starting from the component shown in  FIGS. 3A and 3B . The part produced in this way comprises the blade airfoil  166 , in particular, wherein the upper transverse stay  146  of the reinforcement  144  is arranged in the region of the rotor blade tip  167 . 
         [0043]    The blade airfoil  166  has a relatively complex geometry: its profile resembles that of an aircraft wing. It has a rounded profile nose  168  and a profile rear edge  170 . Extending between the profile nose  168  and the profile rear edge  170  are a concave pressure-side wall and a convex suction-side wall of the rotor blade  120 . A plurality of cooling air ducts (not shown specifically) are introduced in the interior, between the pressure-side wall and the suction-side wall. 
         [0044]    The complex geometry described can be built up in a particularly simple manner by means of 3-D printing, starting from the partially cast rotor blade  120 . The structural weaknesses of the material produced by 3-D printing are compensated by the reinforcement  144 .