Patent Application
20060277753
The present invention relates to methods of repairing metallic blade members and, more generally, metallic blade-shaped members generally having a cross-sectional shape with an effective width that is substantially greater than a thickness thereof. The present invention especially, but not solely, relates to aerodynamic blade members used in turbomachinery such as turbines, aircraft engines, and the like.
Many known devices use bladed members, including turbines, jet aircraft engines, and the like. Moreover, depending on the application, such bladed members are made from metal or special metal alloys, and can be generally expensive to manufacture, maintain, and repair. Yet, over time and in the course of normal operation, these blades are generally subject to damage due to aging and wear. They also occasionally experience damage due to external factors such as foreign object damage in jet engines, problems occurring during the manufacture of constituent parts or during assembly, and design flaws. Damage may also include thermal and mechanical deformation, damage caused by vibration during operation, and damage caused by overheating during operation.
The damage may be in the form of, for example, pits, holes, cracks, and other deformities that may or may not be visible to the naked eye. Such damage must be addressed promptly so that it does not lead to grave problems such as general structural failure, which creates great potential for harm to property and people.
Even though physical damage to blades may be relatively localized, it is common practice to scrap an entire blade with an isolated damaged region rather than repair it. This is of course quite expensive, especially if a significant portion of the blade is still structurally sound.
In addition, with the advent of bladed disks (which are sometimes referred to in the art as “blisks,” and sometimes also known in the art as “bladed monoblock disks”), an inability to perform local repairs becomes very costly in the face of replacing an entire blisk assembly. In general, repair methods that contemplate separating a blade from its hub are by definition incompatible with the blisk structure. As a result, an entire blisk may be scrapped because of damage to just a few of the blades thereon.
Certain repair techniques are conventionally known. For example, U.S. Pat. No. 5,584,662 discloses filling a void formed in a vane by using a braze material similar to that constituting the vane. Laser shock peening is then used to induce deep (i.e., about 508 μm to 1270 μm) compressive residual stresses in the repaired area.
U.S. Pat. No. 6,568,077 also discloses a repair method that includes machining a damaged area using a milling machine to create a notch, followed by filling the notch in a welding step, using a filler material in powder or wire form. Finally, the welded repair is machined to restore proper shape characteristics. Laser shock peening or other after-repair steps are not used according to this document because the shape and size of the notch, and the corresponding welding process, are selected in a way so as to avoid maximum stress regions of the blade.
Finally, U.S. Pat. No. 6,238,187 describes a repair method in which a localized damaged area is cut away from, for example, a turbine blade. A replacement piece is provided that is at least generally similar in size, shape, and mechanical characteristics to the cut-away portion. The replacement piece is welded into place, and is thereafter shaped, if necessary, so as to conform with the overall original blade geometry. Finally, U.S. Pat. No. 6,238,187 discloses that laser shock peening “must” be used to process the weld seam and adjoining regions so as to develop compressive residual stresses in the region that counteract residual tensile fields in the material caused by the welding step.
However, laser shock peening is relatively expensive, time consuming, technically demanding, and usually entails significant startup costs, which can make it unacceptable for some applications, as mentioned in U.S. Pat. No. 6,415,486.
Where laser shock peening is not used to remedy the tensile stress effects of welding, it is also known to subject a repaired part to a thermal treatment followed by shot peening. The counteracting stresses induced as a result however are relatively weak.
In addition, it is known in the conventional art that brazing and other filling methods of repair sometimes suffer because the filler material does not match the mechanical characteristics of the original constituent metal. In addition, this approach means that the process of repair is dependent on the nature of the damage, such that it becomes difficult to establish a readily reproducible repair protocol.
In view of the foregoing, the present invention is generally directed to a method of repairing damage to a metallic blade member that is more economical than the conventional methods discussed above, particularly with respect to the use of laser shock peening. Thus, the present invention includes, generally, cutting away or otherwise removing a damaged portion of the blade member, substituting a replacement portion for the removed portion, welding the replacement portion into place, and burnishing at least the resultant weld seam, if not the entire blade member surface.
Burnishing beneficially cold works the material of the blade member to induce compressive stresses in the repaired blade member. It is a particular feature of the present invention that the burnishing provides an effective but economical substitute for laser shock peening. These compressive stresses counteract residual stresses in the blade member structure caused by welding the replacement portion into place.
Preferably, the replacement portion has material characteristics that correspond closely with the material constituting the blade member so that approximately corresponding mechanical behavior can be realized.
The burnishing can be performed with an appropriate conventional tool, including a tool having, for example and without limitation, a fixed burnishing surface or a moveable burnishing surface. In the latter case, the moveable burnishing surface could be a roller element, such as, but not necessarily, a ball element appropriately supported so as to roll freely.
The present invention will be even better understood with respect to the appended drawings, in which:
It is expressly emphasized here that all of the figures in the application are only examples, and are not limitative. For example, although only a discrete blade member is shown in the figures, the present invention is equally applicable to, for example, bladed disk (“blisk” or “monoblock”) configurations where the blade members are integrally formed with or otherwise fixed to the central disk in a known manner.
In a first preferred embodiment of the present invention, as illustrated in
Damage to airfoil 16 can occur for a variety of known reasons including and not limited to foreign object damage, mechanical fatigue caused by thermal cycling and strong centrifugal forces during operation, defects in design and/or manufacture, and chemical deterioration caused by an operating environment. The forms of damage can also be varied, including without limitation, surface punctures (like at portion 22), edgewise pitting (like at portion 20 at the leading edge), surface pitting, stellate cracks, and elongated cracks. Damage may be visible to the eye, or may be invisible to the eye but detectable by conventional technical inspection methods, including, without limitation, X-ray and sonic inspection.
As indicated in
Regions 20′ and 22′ may be removed by any known method suitable for the materials and work environment in question. Any reference to “cutting away” and the like herein is meant to encompass any removal process appropriate for the structure and constituent material in question, including but not limited to cutting per se. Cutting per se is permissible according to the present invention, but in some cases can have negative side effects on the remaining structure. Another example of a suitable removal process is mechanical milling. Another example of a suitable removal process is cutting by known methods that preferably limit the amount of heat applied to the material. After regions 20′ and 22′ are removed, voids 24 and 26 remain in the blade member 10. It is desirable, according to the present invention, to remove regions 20′ and 22′ in a known, reproducible manner so that the geometries of resulting voids 24, 26 are known. This facilitates the use of replacement portions, as discussed above. Thus, for example, computer numerical control methods can be used in a known manner to control the above-mentioned examples of milling or cutting processes.
Replacement portions 20″ and 22″ are then fixed in place relative to the blade member 10. In a preferred example, replacement portions 20″ and 22″ are fixed in place by an appropriate welding process, such as, without limitation, electron beam welding or gas tungsten arc welding.
If replacement portions 20″ and 22″ are oversized with respect to blade member 10 (as seen in a highly exaggerated form in F
However, welding the replacement portions 20″ and 22″ into place usually creates residual tensile stress in the structure, at least partly due to thermal effects from the welding. These tensile stresses cause weakness in the welded region and susceptibility to fatigue failure. Another source of material weakness is if the replacement portions are made from a material having properties not corresponding with the original blade member material.
It is therefore desirable to induce residual compressive stresses into the structure, e.g. by cold working the material. This cold working beneficially increases fatigue resistance and counteracts the tensile stresses induced during welding. According to the present invention, the cold working is performed by burnishing the weld region and/or portions adjacent thereto in order to induce the desired residual compressive stresses. Here, the mention of “burnishing” according to the present invention includes the more specific known concept of “deep rolling” (i.e. burnishing at relatively high loads, such as, for example, 100 bar to 400 bar (107 pascals (Pa) to 4×107 Pa)). In particular, deep rolling is known to provide significantly more cold working than, for example, laser shot peening.
In one example of the present invention, residual compressive stresses are induced to a depth of about 300 μm to about 1000 μm into the blade member structure. Preferably, the residual compressive stresses are induced to a depth of at least 800 μm.
Accordingly,
The process illustrated in and described with respect to
The shape and size of the portion of the blade member 10 that is removed does not necessarily have to conform to the shape of the damaged portion of the blade member. For example, one or more predefined cutaway shapes can be used, the predefinition of such shapes thereby facilitating the preparation of corresponding replacement portions. It is desirable, nevertheless, to take away as little of the surrounding material as possible, balanced against the recognition that at least part of the blade member 10 adjacent to damaged portions 20 and 22 may have been subjected to difficult or impossible to see damage, such as microcracks or other sub-visible fractures. It is also desirable to take into consideration the stress fields present in the blade member when a damaged portion is removed or cut away, it being most preferable to define regions such as 20′ and 22′ along low stress areas of the blade member 10. The identification of low stress areas in this manner is known in the art.
Finally, it may be desirable according to the present invention to finish the repair process by shot-peening either just the repaired portion of the airfoil 16 or the airfoil 16 overall in a conventional manner in order to restore the surface stresses in the material and to make them relatively uniform.
In some cases, the cumulative extent of damage to a blade member may be judged too extensive for multiple localized repairs as described above with respect to
Here, if the net damage to airfoil 160 is too great to justify (technically and/or economically) multiple localized repairs as discussed above with respect to
More specifically,
The replacement airfoil portion can also be slightly oversized (i.e., thicker, wider, and/or taller) with respect to airfoil stub 240. In such a case, the oversized replacement airfoil portion would be machined in a suitable manner after being welded into place so as to conform with the original blade member configuration, similar to the process discussed and illustrated above in
Once the replacement airfoil portion 260 is welded into place (and machined into proper shape, if needed), a region 280 (
As discussed above, the burnishing or deep rolling performed according to the present invention can be carried out with any known burnishing or deep rolling tool as long as it is able to carry out the desired level of cold work hardening to the appropriate depth. The selected tool may be applied in a non-repeating and/or non-overlapping track on the airfoil surface or it may be applied in a more random manner that may include tracks that overlap each other one or more times. The path along which burnishing or deep rolling is performed may be controlled and/or defined by known tool control methods, including for example, computer numerical control (“CNC”) methods.
The pressure applied by the tool may be constant over the entire region processed, or it may selectively vary in a continuous or in a discontinuous manner. The tool may be provided with a freely moving tip such as a rolling ball or other rolling press member, or it may comprise a non-moving burnishing head, usually rounded in configuration.
Examples of rolling ball burnishing and deep rolling tools are known in the art and are disclosed in, for example, U.S. Pat. No. 6,415,486, U.S. Pat. No. 5,826,453, published U.S. application 2002/0174528, and U.S. Pat. No. 4,947,668. In general, these tools provide a ball member in some form of socket so that the ball is free to roll as the tool is moved relative to the surface being burnished (or more precisely rolled).
In the case of a burnishing tool using a rolling ball member, the force (i.e., load) applied by the tool may depend on several factors, including the diameter of the ball member, the material of the ball member, and the pressure of the fluid supporting the fluid member. The force may also depend on and be controlled by the manner in which the tool is mounted, such as, for example, on a tooling table, a robotic arm, etc.
As mentioned above, it may be desirable to burnish both sides of an airfoil portion at the same time according to the present invention, especially to avoid deformation due to burnishing pressure being applied to only one side of an airfoil. Thus,
| Number | Date | Country | Kind |
|---|---|---|---|
| 04 06467 | Jun 2004 | FR | national |
