Abstract:
A method of reducing crack propagation includes: providing a metallic component having an exterior surface, and using a burnishing element to apply a varying to the exterior surface within a selected area, within which the component has a varying thickness, so as to create a region of residual compressive stress of surrounded by an interior boundary. The distance from the interior boundary to the exterior surface at any location within the selected area is independent of the thickness of the component at that location, and may be controlled by changing the pressure and/or an amount of overlap between burnished segments.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to fatigue-resistant and damage-tolerant components and methods of producing such components. 
         [0002]    Various metallic, ceramic, and composite components, such as gas turbine engine fan and compressor blades, are susceptible to cracking from fatigue and damage (e.g. from foreign object impacts). This damage reduces the life of the part, requiring repair or replacement. 
         [0003]    It is known to protect components from crack propagation by inducing residual compressive stresses therein. Methods of imparting these stresses include shot peening, laser shock peening (LSP), pinch peening, and low plasticity burnishing (LPB). These methods are typically employed by applying a “patch” of residual compressive stresses over an area to be protected from crack propagation, for example a leading edge of a gas turbine engine compressor blade. 
         [0004]    During a burnishing operation, the depth of the compressive residual stress layer can be controlled with process parameters. It is known to control those parameters to transition from high stress areas to low stress areas to prevent a high gradient from compressive to tensile stress fields (this technique is known as “feathering”). However, through the rest of the process, the parameters are held constant, even when processing non-uniform cross-sections (triangular cross-sections, for example). This can result in areas of tensile stresses between layers of compressive residual stress, along with areas where the compressive residual stresses are higher than the intended range. 
         [0005]      FIG. 1  illustrates a generic metallic component  10  with a surface  12 . A burnishing element  14  is pressed against the surface under substantial pressure and translated along a selected path. In this example the burnishing element  14  is a sphere, but cylindrical rollers are also used. Typically a pressurized fluid is used to force the burnishing element  14  onto the surface  12  of the component  10 . Mechanically loaded tools are also used. Appropriate equipment, of a known type, typically CNC controlled, is provided to load the burnishing element  14 , and to move it along the desired path. The pressing force used during burnishing is such that it induces plastic strain and a region of residual compressive stresses  16  within the component  10  near a burnished area  18 . A region of residual tensile stresses  17  exists around the border of the region  16 . 
         [0006]      FIG. 2  illustrates an exemplary gas turbine engine compressor blade  20 . This component is used merely as an example of a part to which both prior art methods and the present invention may be applied. the present invention is equally applicable to other types of components susceptible to cracking from fatigue or damage, such as compressor stator vanes, fan blades, turbine blades, shafts and rotors, stationary frames, actuator hardware and the like. Such components may be made from metal alloys, ceramics, or composite materials (e.g. carbon fiber composites). The compressor blade  20  includes an airfoil  22 , a platform  24 , and a shank  26 . In this particular example the shank  26  includes a dovetail  28  for being received in a slot of a rotating disk (not shown). The airfoil  22  has a leading edge  30 , a trailing edge  32 , a tip  34 , a root  36 , a pressure side  38 , and a suction side  40  opposite the pressure side  38 . A burnishing tool  42  carrying a burnishing element  14  is shown tracing out a selected burnishing path “p” along the surface of the airfoil  22 . In this example, the path “P” includes a plurality of linear segments  23  arranged in a series of S-turns. The path has a footprint with a width “W” determined by the width of the burnishing element  14  and the applied pressure. The linear segments  23  are separated by an step-over distance “S”. In cases where the step-over distance S is less than the width W, overlap of the segments  23  will occur. In most applications, there will be substantial overlap to achieve adequate coverage and desired stress profiles. 
         [0007]      FIGS. 3A and 3B  illustrate a prior art burnishing treatment being applied to edge  32  of the airfoil  22 .  FIG. 3A  shows the treatment being applied to the pressure side  38  by a single burnishing element  14 , while the airfoil is supported by a block  44 . In this case, a constant applied pressure in the normal direction “f” is selected to generate a region  46  of residual compressive stress which has depth “d” defined as a distance from the surface of the pressure side  38 , expressed as a fraction of the total thickness of the airfoil  22  at the point of measurement. The burnishing element  14  is moved from left to right. The depth d will decrease substantially as the burnishing element  14  traverses the thicker portion of the airfoil  22  distal from the trailing edge  32 . The result is that the interior boundary  48  of the region  46  is not parallel to a mid-chord plane M of the airfoil  22 . Under these circumstances, the depth d will vary significantly from a desired magnitude at opposite axial ends of the region  46 , regardless of which end is used as the basis for setting the applied pressure. 
         [0008]      FIG. 3B  illustrates the prior art burnishing treatment being applied to both the pressure side  38  and the suction side  40  of the airfoil  22  by opposed burnishing elements  14  and  14 ′. In this case, the applied pressure in the normal directions, denoted f and f′, are selected to generate regions  50  and  52  of residual compressive stress which have depths d and d′ measured from the surface of the pressure side  38  and suction side  40 , respectively, and expressed as a fraction of the total thickness of the airfoil  22  at the point of measurement. The depths d and d′ are typically chosen to generate through-thickness residual compressive stress near the trailing edge  32 . However, as shown, the depths d and d′ will decrease substantially as the burnishing elements  14  and  14 ′ traverse the thicker portion of the airfoil  22  distal from the trailing edge  32 . The result is that the interior boundaries  54  and  56  of the regions  52  and  54  are not parallel to a midplane M of the airfoil  22 . If the pressures f and f are just enough that through-thickness residual compressive stress is produced near the trailing edge  32 , this results in an internal region  58  of residual tensile stress at thicker portions of the airfoil  22 . It is possible to select the pressures f and f so that the regions  50  and  52  merge to produce through-thickness residual compressive stress, even at the thickest portion of the treated area. However, this would result in excessive compressive stress levels near the trailing edge  32 , because of overlap of the regions  50  and  52 . It could also damage the airfoil  22  and result in undesired deformation. 
         [0009]    In light of the above shortcomings of the prior art, there is a need for a method of producing uniform through-thickness residual compressive stresses in components of variable thickness. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The above mentioned need is met by the present invention, which provides a method for varying the parameters of a burnishing operation in consideration of the workpiece thickness so that a desired penetration depth of residual compressive stress is achieved regardless of local thickness. 
         [0011]    According to one aspect, the invention provides a component having at least one exterior surface, the component including at least one region of residual compressive stress extending inwards from the surface in at least one selected area within which the thickness of the component varies, the region surrounded by an interior boundary. 
         [0012]    According to another aspect of the invention, an airfoil for a gas turbine engine includes a root spaced apart from a tip, spaced-apart leading and trailing edges, a suction side extending from the leading edge to the trailing edge, and an opposed pressure side extending from the leading edge and the trailing edge. A thickness of the airfoil is defined between the pressure side and the suction side; and a first region of residual compressive stress extending inward from a first area of a selected one of the pressure side and the suction side. The thickness of the airfoil varies within the first area, and the first region is surrounded by a first interior boundary, A second region of residual compressive stress extends inward from a second area of a the other one of the pressure side and the suction side, the thickness of the airfoil varying within the second area, wherein the second region is surrounded by a second interior boundary. Substantially all of the first and second interior boundaries are blended together. 
         [0013]    According to another aspect of the invention, a method of reducing crack propagation in components includes: providing a component having an exterior surface; and using a burnishing element to apply a varying pressure to the exterior surface within a selected area, within which the component has a varying thickness, so as to create a region of residual compressive stress surrounded by an interior boundary; wherein the distance from the interior boundary to the exterior surface at any given location within the selected area is independent of the thickness of the component at that location. 
         [0014]    According to another aspect of the invention, a method of reducing crack propagation in components includes providing a component having opposed, spaced-apart first and second exterior surfaces; and using first and second burnishing elements to apply a varying pressure to the exterior surfaces within respective first and second selected areas, within which the component has a varying thickness, so as to create first and second regions of residual compressive stress surrounded by first and second interior boundaries. The distance from each of interior boundaries to the respective exterior surface at any given location within the respective selected area is independent of the thickness of the component at that location. 
         [0015]    According to another aspect of the invention, a method of reducing crack propagation in components, includes: providing a component having opposed, spaced-apart first and second exterior surfaces; and using a first burnishing element to apply a pressure to the first exterior surface within a first selected area, within which the component has a varying thickness, while moving the first burnishing element along a first preselected path including segments separated by a step-over distance, so as to create a first region of residual compressive stress surrounded by a first interior boundary; wherein the step-over distance is selected to control an amount of overlap between adjacent segments, consequently changing the distance from the first interior boundary to the first exterior surface, such that the distance from the interior boundary to the first exterior surface at any given location within the first selected area is independent of the thickness of the component at that location. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0017]      FIG. 1  is a schematic side view of a prior art burnishing process being applied to a surface of a component; 
           [0018]      FIG. 2  is a schematic perspective view of a prior art burnishing process being applied to a gas turbine engine compressor blade; 
           [0019]      FIG. 3A  is a schematic side view of a prior art burnishing treatment being applied to a single side of the compressor blade of  FIG. 2 ; 
           [0020]      FIG. 3B  is a schematic side view of a prior art burnishing treatment being applied to both sides of the compressor blade of  FIG. 2 ; 
           [0021]      FIG. 4A  is a schematic side view of a burnishing treatment as described herein being applied to a single side of a compressor blade; 
           [0022]      FIG. 4B  is a schematic side view of a burnishing treatment as described herein being applied to both sides of a compressor blade; and 
           [0023]      FIG. 5  is a side view of a burnishing treatment as described herein being applied to a component of variable thickness. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIGS. 4A and 4B  illustrate an exemplary burnishing treatment in accordance with an aspect of the invention being applied to the trailing edge region of an airfoil  122 , which before treatment is identical to the airfoil  22  described above.  FIG. 4A  shows the treatment being applied to the pressure side  138  within a selected area by a burnishing element  114 , while the airfoil  122  is supported by a block  144 . The treatment described herein may be applied to any portion of the airfoil  122 . In this case, the applied pressure in a direction normal to the surface, indicated at F, is selected to generate a region  146  of residual compressive stress which has a depth D (this could also be described as penetration) measured from the surface of the suction side  138 , and expressed as expressed as a fraction of the total thickness of the airfoil  122  at the point of measurement. To achieve a more uniform depth D, the burnishing parameters are changed as the burnishing element  114  moves to areas of different thicknesses. Specifically, as the burnishing element  114  is moved from a position near the trailing edge  132  to a thicker portion of the airfoil  122  distal from the trailing edge  132 , the pressure F in the normal direction is increased. The pressure is generally proportional to the thickness. Using this varying pressure, it is possible to generate a region  146  which has an interior boundary  148  with a selected profile. The interior boundary  148  may be made parallel to an arbitrary preselected interior plane. In the illustrated example, a substantial portion of the interior boundary  148  is substantially parallel to, and coincident with, a midplane M of the airfoil  122 . 
         [0025]    The depth D may also be manipulated to control the interior boundary  148  in whole or in part by controlling the amount of overlap between burnished segments as the burnishing element  114  is moved through a selected path. For example, if the step-over distance (denoted “S” in  FIG. 2 ) is greater than the burnished segment width “W”, there will be no overlap. As the step-over distance is decreased to less than the width “W”, the overlap increases. The greater the overlap, the greater the depth D will be. This is true even when the applied pressure is held constant, although the effect on depth D of overlap alone is thought to be less than that of the burnishing pressure, 
         [0026]      FIG. 4B  illustrates an exemplary burnishing treatment in accordance with another aspect of the invention being applied to both the pressure side  138  and the suction side  140  of the airfoil  122  within selected areas thereof by opposed burnishing elements  114  and  114 ′. In this case, the applied pressures in the normal directions, indicated at F and F′ are selected to generate regions  150  and  152  of residual compressive stress which have depths D and D′ measured from the surface of the pressure side  138  and suction side  140 , respectively, and expressed as a fraction of the of the total thickness of the airfoil  122  at the point of measurement. This depths D and D′ are chosen so that substantially all of their interior boundaries  154  and  156  are blended together at a midplane M of the airfoil  122 . Substantially all of, or portions of, the interior boundaries  154  and  156  may be coincident with each other. This results in the generation of through-thickness residual compressive stress in the selected areas without exceeding desired compressive stress levels. As noted above, the interior boundaries  154  and  156  may have arbitrary preselected profiles and may be made parallel to arbitrary, preselected interior planes. The area of residual tensile stress  58  described above with respect to the prior art method is eliminated. 
         [0027]    The depths D and D′ may also be manipulated to control the interior boundaries  154  and  156  in whole or in part by controlling the amount of overlap between burnished segments as the burnishing elements  114  and  114 ′ are moved through selected paths, as described above with respect to the single burnishing element  114 . 
         [0028]      FIG. 5  illustrates the another exemplary burnishing treatment in accordance with an aspect of the invention being applied to a surface  238  of a component  222  within a selected area by a burnishing element  214 . In this case, the surface  138  includes at least one feature  139  (such as a ridge or groove) which extends significantly above or below the remainder thereof. The applied pressure F in the normal direction is varied as described above to generate a region  250  of residual compressive stress which has a varying depth D″ measured from the surface  238  and expressed as a fraction of the total thickness of the component  222  at the point of measurement. Using this varying pressure, it is possible to give the interior boundary  254  a selected profile. The interior boundary  254  may be made parallel to an arbitrary preselected interior plane. In this case, the depth D″ is varied such that substantially all of the interior boundary of  254  of the region  250  is substantially parallel to the surface  238 . 
         [0029]    The pressure variation described above may be achieved in various ways. For example, the pressure could be manually varied by operator control as the burnishing element traverses different portions of the workpiece. However, as the motion of the burnishing element is typically CNC-controlled, it is possible to analyze the dimensions of the workpiece and based on those dimensions, generate and store a data “map” relating desired pressure to identifiable coordinates points on the workpiece. The pressure on the burnishing element would then be automatically varied by the burnishing equipment based on reference to the map as the burnishing equipment moves the burnishing tool through a selected path having segments separated by a step-over distance as described above. In addition, the step-over may be controlled either to manipulate the overlap between segments when using a constant pressure, as described above, or to hold a selected amount of overlap constant throughout the process, since the width of the burnished segment varies with varying pressure. For example, if the burnishing pressure is increased, causing an increase in the width of the burnishing line, the control would correlate the increased pressure to the resulting increased with and the step-over distance for the next segment would be decreased so that the overlap is not undesirably increased. 
         [0030]    The foregoing has described fatigue- and damage-resistant components and methods for making such components. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.