Abstract:
A method of surface treating a dovetail in a blade includes the steps of providing a burnishing operation along circumferential sides of the dovetail. Next, shot peening is provided at axial ends of the dovetail. A fan blade made by the method is also claimed.

Description:
BACKGROUND 
       [0001]    This application relates to a surface treatment method for treating a dovetail in a blade. 
         [0002]    Gas turbine engines are known, and typically have a fan delivering air to a compressor. From the compressor, the air passes downstream into a combustion section where it is mixed with fuel and burned. Products of that combustion pass downstream over turbine rotors which in turn drive the fan to rotate. 
         [0003]    The fan may include a plurality of separate blades mounted into a rotor. The blades have an airfoil which moves the air, and a radially inner portion, known as a dovetail, which is received within a slot in the rotor. 
         [0004]    The dovetail is subject to high stress during operation of the gas turbine engine. Thus, it presents challenges to a designer of a fan blade. 
         [0005]    One method that has been proposed to address the stresses applied into the dovetail is to utilize a burnishing operation on the dovetail. A burnishing operation will induce stresses within the part. These induced stresses are residual compressive stresses that can be induced into the part to a desired depth. The induced stresses counter applied stresses during operation to result in an overall lower stress level. 
         [0006]    The burnishing operation does provide good control over the level of induced stresses and the depth into the material. However, burnishing typically requires a somewhat flat surface, and thus, complex surfaces may not be candidates for burnishing. 
         [0007]    Once a burnishing operation has been applied to a part, surfaces that have not been burnished may have increased stresses or other changes which may be undesirable. 
       SUMMARY 
       [0008]    A method of surface treating a dovetail in a blade includes the steps of providing a burnishing operation along circumferential sides of the dovetail. Next, shot peening is provided at axial ends of the dovetail. A blade made by the method is also claimed. 
         [0009]    These and other features of this application will be better understood from the following specification and drawings, the following of which is a brief description: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  shows a fan blade. 
           [0011]      FIG. 1B  shows the fan blade mounted into a rotor. 
           [0012]      FIG. 2  shows a first method step according to this invention. 
           [0013]      FIG. 3  shows a subsequent step. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1A  shows a fan blade  20  having an airfoil  18  extending radially outwardly of a dovetail  24 . The airfoil can be said to have a leading edge  21  and a trailing edge  22 . The dovetail  24  has circumferential side surfaces  100 , and axial ends  102  and  104 . As can be appreciated from  FIG. 1A , the axial ends  102  and  104  are not regular surfaces, but have complex curved shapes. The circumferential sides  100  have generally regular shapes along an axial length. 
         [0015]      FIG. 1B  shows the fan blade  20  mounted within a slot  17  in a rotor  16 . As can be appreciated, the sides  100  receive a good deal of stress when the rotor  16  is rotating, as the rotational force is transmitted through the sides  100 . The ends  102  and  104  do not receive the same forces. As shown, the circumferential sides  100  have a more regular surface than the ends  102  and  104 . 
         [0016]    In order to surface treat the fan blade dovetail to withstand such forces, a first surface treatment step, burnishing, is shown in  FIG. 2 . The term burnishing can be generally described as bringing a tool into contact to physically treat surfaces on a workpiece. 
         [0017]    A burnishing operation with a burnishing tool, shown rather schematically at  50 , has surfaces  104  brought into contact with the side surfaces  100 . The burnishing tool  50  may provide low plasticity burnishing. One known low plasticity technique is disclosed in U.S. Pat. No. 6,415,486. Other burnishing operations can be used however. 
         [0018]    As shown in  FIG. 2 , in a generic burnishing operation, the tool  50  is controlled with the computer numerical control (CNC) positioning control  56 , and a pressure control  58 . Broadly speaking, once the selected area  100  of the dovetail has been identified, a desired magnitude of compression from the tool surfaces  52  and a residual desired stress distribution are determined. The pressure is controlled against the surfaces to form zones of deformation having a deep layer of compressive stress. The pressure can be varied to achieve a desired residual stress distribution, and a magnitude of compression within the dovetail  24 . 
         [0019]    While burnishing works very well on the circumferential side surfaces  100 , it is not as effective for the ends  102  and  104 . Since the ends  102  and  104  have complex surfaces, they are not as easily treated with the burnishing technique. In addition, the burnishing of the sides  100  does result in undesirable stress distribution at the ends  102  and  104 . 
         [0020]    Thus,  FIG. 3  shows a step subsequent to the  FIG. 2  step. In  FIG. 3 , shot peening masks  62  are mounted to enclose the ends  102  and  104 . Shot peening tool  60  delivers particles  64  against the ends  102  and  104 . The shot peening process is shown somewhat schematically in  FIG. 3 , however, a worker of ordinary skill in the art would recognize how to provide shot peening to the surfaces  102  and  104 . 
         [0021]    After the two-step method is complete, the fan blade  20  has been surface treated at both of its ends  102 / 104 , and the sides  100  of the dovetail such that it will provide effective operation in the challenging environment of the gas turbine engine. 
         [0022]    In addition, as an initial optional step prior to the  FIG. 2  step, the entire dovetail may be initially shot peened prior to the burnishing. 
         [0023]    The blade as disclosed above is for use as a fan blade. It is believed the teachings of this application would have most success when utilized on blades that will exist in lower temperature portions of a gas turbine engine. However, it is possible that blades in higher temperature regions may also benefit from the methods. In addition, the methods may have application in blades having air foils and dovetails for use in applications other than gas turbine engines. Thus, while the invention might have most application in fan blades and lower pressure compressor sections, it may also have application in other locations. 
         [0024]    Although an embodiment has been disclosed, a worker of ordinary skill in the art would recognize that certain modification would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.