Patent Application
20070281088
Like reference numbers and designations in the various drawings indicate like elements.
The exemplary deposition is of nominal Ti-6Al-4V condensate atop a like substrate. Alternate depositions may include Ti-6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-1V. The deposition may be from a melted ingot at least partially through a pool containing one or more refractory or other elements, which may be essentially non-consumed during deposition (e.g., a pool formed from a 30% Mo-70% Zr mixture). Accordingly, the droplets may tend to have compositions similar to the surface layers of the pool. In the absence of the non-consumed pool additive, the droplet 28 might have a similar composition to the ingot yet still produce similar defects. Many droplets in systems using an Mo-containing pool would have Mo concentrations of at least 10% by weight; others at least 20%. This may be somewhat less than the Mo percentage of the non-expending pool material to reflect possible dilution by deposition material elements in the pool.
In the exemplary implementation, the substrate 28 has an α-β microstructure of medium to coarse grains (e.g., 10-40 μm characteristic grain size (e.g., mean) or about ASTM 10.5-6.5). An exemplary 10-20% by weight of the substrate is β phase with the remainder essentially a phase. The condensate matrix (away from the defects) also has an α-β microstructure but of very fine grains (e.g., acicular a grains of 5-10 m in length and 2-5 μm in thickness, lengthwise oriented along the condensate growth/deposition direction). The trunk size will depend, in substantial part, upon the droplet size. Exemplary trunk diameters are from about 20 μm to about 50 μm. However, much larger trunks are possible. The trunks have a columnar α-β microstructure. This microstructure may have a characteristic grain size several times greater than that in the matrix and the grains may be elongated in the direction of accumulation (i.e., away from the substrate). Particularly in the case of very large diameter trunks (e.g., in excess of 100 μm in diameter), there may be porosity around the trunk. The grain discontinuity at the trunk-matrix interface and the particular alignment of trunk grains may cause structural weaknesses affecting, inter alia, ductility, fracture toughness, fatigue resistance, fretting fatigue resistance, corrosion resistance, wear resistance, crack nucleation resistance, and the like.
According to the present invention, the condensate is subjected to a burnishing process to mitigate one or more of these structural weaknesses. 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. Prevey, 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. Prevey, 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.
An exemplary part is a blade 40 (
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 or at the tip.
To address distributed erosion, the condensate (coating) 20 may be applied to a zone 72. An exemplary zone 72 extends along substantially an entirety of the airfoil along the pressure and suction sides to a boundary 74. The exemplary boundary 74 is a radial distance S1 from the tip. Exemplary S1 is more than 50% of S along the entire chord.
For an exemplary erosion restoration 100 (
In the exemplary method, a finish machining 106 of the condensate may locally bring the final airfoil contour within specification. Exemplary machining 106 involves use of an abrasive belt sander with hand manipulation of the blade relative to the sander. Alternative machining by automated blending processes is also known. An inspection (e.g., probe or laser scan, not shown) may verify dimensional compliance of the part.
The condensate is subjected to a burnishing 108. Exemplary burnishing is by 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. Alternatively, the burnishing may be performed before or in the absence of the finish machining 106.
The exemplary burnishing is over essentially the entire condensate with slight overlap onto the uncovered 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 as-applied condensate has a median/modal thickness of 0.008 inches (more broadly up to about 0.015 inch, and more narrowly 0.004-0.008 inch). An exemplary burnishing imparts a residual compressive stress over a depth zone extending to or slightly below the condensate-substrate interface at the surface 22. An exemplary depth zone is 1.0-2.0 times the local coating thickness.
An exemplary residual stress in the 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 condensate. Below the zone, the residual stress will drop off. The exemplary stress may be at least 20 ksi at the condensate/substrate interface. Below the interface the stress will further drop. The stress may be below 20 ksi and, more narrowly, may reach zero within an exemplary 0.001-0.005 inch (more narrowly 0.001-0.004 inch) below the interface, especially for coatings machined post burnishing or coatings where pre-burnishing machining has not substantially reduced coating thickness. Depending on coating thickness and the timing and extent of such machining, this location (below 20 ksi and, more narrowly, zero) may be within an exemplary 0.015 inch (more narrowly, 0.012 inch (e.g., 0.009-0.012 inch for no or minimal post-burnish machining)) of the condensate surface. Such a shallow depth of stress distribution may limit distortion of the part.
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 covered by the condensate and subject to the burnishing. For example, there may be a shot peening 110. 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.
Among possible variations are reorderings of the burnishing, machining, and/or shot peening.
The deposition may cover the shrinkage zone, with slight overlap onto unaffected areas. An alternative deposition may be broader, (e.g., covering a similar area to that of the deposition 104). Deposition thickness may be similar to that of the deposition 104. A post-deposition machining 134 (e.g., also similar to 106) may be performed.
Burnishing 136 (e.g., similar to 108) may cover the condensate with slight overlap onto unaffected areas or may be of a broader extent such as that of the burnishing 108. Shot peening 110 may similarly follow. As with the first example, burnishing parameters may be determined by appropriate testing.
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 deposition, welding, machining, 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 Ti-based coatings may be on other areas and other components. Other blade examples involve restoration of worn or fretted blade roots. Accordingly, other embodiments are within the scope of the following claims.
