Abstract:
A gas turbine engine blade has a platform. A root depends from the platform and an airfoil extends from the platform. The airfoil has leading and trailing edges and pressure and suction sides. The blade has a substrate having a surface. A compressive stress exists below a first region of the surface. The first region extends over a majority of a streamwise perimeter of the airfoil at a location at a spanwise distance from the tip or more than 50% of a tip-to-platform span. A coating is on the surface including at the location.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application of Ser. No. 11/499,163, filed Aug. 3, 2006 now abandoned, and entitled Pre-Coating Burnishing of Erosion Coated Parts, now abandoned, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to gas turbine engines. More particularly, the invention relates to erosion coated blades. 
     Compressor blades operating in modern aircraft gas turbine engines are subject to erosion damage. The damage may be caused by ingestion of dirt and sand (e.g., from airport runways). Erosion damage shortens the operating life of blades because of changes in aerodynamic characteristics that reduce performance and reduce fatigue life. 
     One approach to extending blade life is to apply erosion-resistant coatings such as Co—WC and NiCr—CrC to those areas of the blade airfoil that are subject to the greatest erosion. However, these coatings have a deleterious effect on the fatigue properties of blade substrates, typically titanium and nickel based alloys. Consequently, the erosion coating is only applied to those areas of the airfoil where operating stresses do not exceed the reduced blade fatigue strength. It would be desirable to have ways to mitigate the deleterious effect erosion coatings have on the fatigue properties of the substrate. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention involves a gas turbine engine blade. The blade has a platform. A root depends from the platform and an airfoil extends from the platform. The airfoil has leading and trailing edges and pressure and suction sides. The blade has a substrate having a surface. A compressive stress exists below a first region of the surface. The first region extends over a majority of a streamwise perimeter of the airfoil at a location at a spanwise distance from the tip of more than 50% of a local tip-to-platform span. A coating is on at least a portion of the first region of the surface including at the location. 
     Another aspect of the invention involves a method for making an article of a metallic material. The article comprises a substrate and an erosion coating. An extent of a region of the substrate is selected to be burnished to mitigate a fatigue effect of the coating. A roller deformation is performed on the region, then the coating is applied. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a blade. 
         FIG. 2  is a flowchart of a first process for manufacturing or remanufacturing the blade. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary blade  40 . The blade has an airfoil  42 , a platform  44 , and an attachment root  46 . The airfoil has a leading edge  48 , a trailing edge  50 , and pressure and suction sides  52  and  54  extending between the leading and trailing edges. The airfoil extends from an inboard end  56  at the platform outboard surface  58  to an outboard end or tip  60 . The root depends from an underside  62  of the platform  44  and may have a convoluted profile (e.g., so-called dovetail or fir tree profiles) for securing the blade  40  to a complementary slot of a disk (not shown). A local span S is the radial distance between the tip  60  and the airfoil inboard end  56 . The span S will vary along the airfoil chord. 
     An exemplary airfoil may be subject to one or more forms of wear and/or damage. Wear may include widely distributed erosion. Damage may include nicks and chips from foreign object damage (FOD), usually near the leading edge  48  or at the tip  60 . Erosion typically is concentrated along an outboard portion of the airfoil (e.g., along a zone  66 ). An exemplary zone  66  extends along a slight majority of the airfoil span along the pressure and suction sides from the tip  60  to boundary  68 . Within this zone, the heaviest erosion will typically be along the outboard third of the airfoil (i.e., the third of the airfoil closest to tip  60 ). The exemplary boundary  68  is a radial distance S 1  from the tip. Exemplary S 1  is slightly more than 50% of S along the entire chord. The actual extent of erosion damage is influenced by both the airfoil design and the operating characteristics of the engine. 
     To provide erosion resistance, the metallic blade substrate may have a coating  70  along a zone  72 . Exemplary Ti-based substrate alloys include Ti—6Al—4V, Ti—8Al—1Mo—1V, Ti—6Al—2Sn—4Mo—2Zr, Ti—6Al—2Sn—4Mo—6Zr, and Ti—5.5Al—3.5Sn—3Zr—1Nb. Other substrates include, but are not limited to, stainless steels (e.g., 17-4PH) and Ni-base superalloys (e.g., alloy 718). Exemplary coatings include Co—WC, NiCr—CrC, Ti—N, and Ti—AlN. An exemplary zone  72  extends along a majority of the airfoil along the pressure and suction sides from the tip  60  to boundary  74 . The exemplary boundary  74  is a radial distance S 2  from the tip. Exemplary S 2  is more than 50% of S along the entire chord and more than S 1 . The coating may extend beyond the heaviest erosion areas and provide enhanced foreign object damage. 
     The coating  70  may have deleterious effects upon blade structural properties (e.g., fatigue properties). Traditionally this has limited use of erosion coatings to areas of the blade airfoil that experience relatively low stresses. According to the present invention, the blade is subjected to a burnishing process to mitigate one or more of these effects and permits coating application over more of the blade airfoil. The exemplary burnishing process is a low plasticity burnishing process. 
     Low plasticity burnishing of aerospace parts is discussed in U.S. Pat. Nos. 5,826,453, 6,672,838, and 6,893,225 and Published Application No. 2005-0155203. Use of such burnishing for Ti-based parts is also discussed in P. Prevéy, N. Jayaraman, and R. Ravindranath, “Use of Residual Compression in Design to Improve Damage Tolerance in Ti-6Al-4V Aero Engine Blade Dovetails,” Proc. 10th Nat. HCF Conf., New Orleans, La., Mar. 8-11, 2005 and P. Prevéy, N. Jayaraman, and J. Cammett, “Overview of Low Plasticity Burnishing for Mitigation of Fatigue Damage Mechanisms,” Proceedings of ICSP 9, Paris, Marne la Vallee, France, Sep. 6-9, 2005. 
     Before erosion coating application, the blade airfoil may be subject to a low plasticity burnishing (“LPB”) process along a zone  80 . An exemplary zone  80  extends along a majority of the airfoil along the pressure and suction sides from the tip  60  to boundary  82 . The exemplary boundary  82  is a radial distance S 3  from the tip. Exemplary S 3  is more than 50% of S along the entire chord and more than S 2  and S 1 . 
     Compared with an unburnished blade, the burnishing may allow the coating  70  to be applied over a greater radial span from the tip  60  toward the platform  44 . For example, in a prior art process, S 2  may be relatively small (e.g., less than S 1  and less than 50% of S). For example, blades in models of the CFM56 compressor employ an erosion coating that extends approximately 30-40% of the span from the tip, whereas blades of various other engine families do not use erosion coatings at all. 
     Although burnishing of tip areas is known, the nature of the present burnishing may have one or more differences relative to that of conventional wisdom in the art. Applied without appropriate consideration of the erosion coating, prior art burnishing may be over a much smaller extent of the airfoil. The prior art may involve a smaller radial span from the tip and may be more localized near the leading edge (to address foreign object damage rather than operating stresses on the blade). The burnishing that addresses foreign object damage (FOD) in uncoated blades is more likely to be deeper than that for use in conjunction with erosion coatings. It may, however, be possible to combine deeper burnishing of tip and/or leading edge portions with shallower burnishing of more proximal and/or trailing/downstream portions. 
       FIG. 2  shows an exemplary burnishing and coating process  100 . The process may receive a clean blade substrate. The substrate is subjected to a burnishing  102 . Exemplary burnishing is by a fluid rolling elements. Exemplary rolling elements are spheres/balls. Single point burnishing and opposed two-point caliper burnishing are disclosed in the references cited above. 
     The exemplary burnishing  102  is over essentially the entire region to be coated with slight overlap onto the adjacent area of the airfoil. The exemplary burnishing is shallow (i.e., imparting residual compressive stress not extending through the entire thickness/depth of the substrate). An exemplary burnishing imparts a residual compressive stress over a depth zone of up to about 0.015 inch (e.g., more narrowly 0.004-0.008 inch). 
     An exemplary residual stress in the depth zone has a peak value of 100-110 ksi for Ti-6-4, more broadly 90-120 ksi. The upper end of the range may be limited by the strength of the substrate. Below the depth zone, the residual stress will drop off. Such a shallow depth of stress distribution may limit distortion of the part. In another example, residual stress in alloy 718 blades may approach 150 ksi due to the higher yield strength of this material. 
     LPB results in significantly less cold work than more traditional processes, such as shot peening. This is of particular relevance in the present use. The Prevéy, et al. references cited above have demonstrated enhanced fatigue properties of LPB after exposure to temperatures as high as 600 C. This becomes significant when one considers the substrate heating associated with compressor blades operating at temperatures exceeding 500 C. Peening stresses tend to diminish with time at intermediate temperatures, thereby negating their beneficial effect. LPB stresses are associated with lower levels of cold work that appear to be more stable at these same temperatures. Thus, LPB is advantageous relative to shot peening in the erosion coating area. 
     The burnishing parameters needed to provide the desired stress distribution may be developed through an iterative destructive testing process. In an exemplary testing process, a localized inspection process (e.g., x-ray diffraction) may be used to evaluate the depth and magnitude distribution of the residual stress and may indicate the need for altering the burnishing parameters for the location. 
     In the exemplary method, there may be a further mechanical treatment of the areas of the blade beyond those to be covered by the coating and subject to the burnishing. For example, there may be a shot peening  104 . The shot peening may address the attachment root  46 . The shot peening may provide a stress distribution that is shallower, but of higher peak compression than the burnishing. 
     The coating may be applied  106  (e.g., by high velocity oxy-fuel (HVOF) or other high energy thermal spray process, or by physical vapor deposition (PVD)). 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the invention may be implemented as a modification of or using various existing coating, burnishing, and other techniques and apparatus. Also, various boundary and transition areas may have properties departing from those discussed above. Although illustrated as applied to a blade airfoil, the erosion coatings may be on other areas and other components. Accordingly, other embodiments are within the scope of the following claims.