Patent Application
20170274451
The invention relates to forming inner contours of a component for a used in a gas turbine engine such as a blade or vane using an electrochemical machining process.
Investment casting may be used to produce hollow parts having internal cooling passages such as blades and vanes used in gas turbine engines. During the conventional investment casting process wax is injected into a wax cavity to form a wax pattern between a core and a wax die. The wax die is removed and the core and wax pattern are dipped into the ceramic slurry to form a ceramic shell around the wax pattern. The wax pattern is thermally removed, leaving a mold cavity. Molten metal is cast between the ceramic core and the ceramic shell, which are then removed to reveal the finished part.
Any movement between the ceramic core and the wax die may result in a distorted wax pattern. Since the ceramic shell forms around the wax pattern, and the ceramic shell forms the mold cavity for the final part, this relative movement may result in an unacceptable part. This is particularly so for thin-walled components, where a shift may change the wall thickness by a relatively large percentage. Likewise, any movement between the ceramic core and the ceramic shell when casting the airfoil itself may result in an unacceptable part. This nature of the investment casting process, where two discrete parts must be held in a single positional relationship during handling and multiple casting operations, makes holding the tolerances difficult.
In order to overcome this relative shifting, U.S. Pat. No. 5,296,308 to Caccavale et al. describes one approach where a ceramic core has bumpers on the ceramic core that touch, or almost touch, the wax die during the wax pattern pour. This controls a gap between the ceramic core and the wax die, and likewise controls a gap between the ceramic core and the ceramic shell. Controlling the gap minimizes shifting between the ceramic core and the ceramic shell, and this improves control of the wall thickness of the airfoil. The bumpers are positioned at key stress regions to counteract distortions. However, the final part may have a hole where the bumpers were located, between an internal cooling passage and a surface of the airfoil, which allows cooling fluid to leak from the internal cooling passage. Consequently, there remains room in the art for improved methods of forming
The invention is explained in the following description in view of the drawings that show:
The present inventors have developed a novel way to form a gas turbine engine component such as a blade or vane. The component may be a thin-walled component such as an airfoil having cooling channels under a thin exterior wall. The inventors propose to form a pilot channel in the component and then enlarge the pilot channel until a final interior contour of the component is formed using electrochemical machining (ECM). An ECM electrode will be lowered into the pilot channel and excess material may be removed from beneath a tip of the electrode as it is lowered as well as from locations astride the tip of the electrode when being lowered and/or when stationary. The electrode may be shaped or masked to form desired inner contours and may be masked to form flow-interrupting elements inside the channel being formed. The pilot channel may be formed as part of a casting operation. In such an instance the component is cast around a core to form a cast monolith having the pilot channel formed therein. Alternately, the pilot channel may be machined via conventional techniques such as drilling, electro discharge machining, or electrochemical machining. When the pilot channel is formed via ECM, the entire interior volume of the component may be formed via ECM. The component to be electro machined may be cast or formed through any methods known to those in the art. The component to be machined may alternately be formed without a pilot hole. For example, a component may be cast without a pilot hole and then all interior machining is carried out via ECM.
Twisted airfoils present challenges for conventional machining processes such as drilling because drilling produces straight holes/channels with a circular cross section having a single diameter (per drilling operation). Consequently, drilled holes cannot follow the twist of the airfoil nor can they create the complicated inner contours (e.g. shapes) preferred. However, the process disclosed herein is compatible with twisted airfoils (as well as non-twisted airfoils) because the electrode itself can be twisted and because the electrode can be shaped and/or masked to machine various contours.
In the exemplary embodiment of
For example, a core used to form a varying diameter pilot channel 40 may be narrower at its base and/or tip, and wider in its middle if the channel to be formed follows this general shape. This way the middle region would take less time to electrochemical machine when compared to a pilot channel 40 of constant diameter.
In the exemplary embodiment shown the pilot channel 40 will first be enlarged using a first electrode 42 that will enter the pilot channel 40 at a first location (e.g. at one end of the pilot channel 40). The first electrode 42 will be use in the electrochemical machining process to form an intermediate channel having an intermediate channel contour 44 that represents a surface of the intermediate channel once it is formed. In the exemplary embodiment the first electrode 42 may be lowered from the tip 20 of the airfoil 14 toward the platform 12 of the airfoil 14, and may continue through the base of the airfoil. The tip 20 is smaller and channel formation at the tip 20 may require greater process control, and greater process control occurs with electrochemical machining when a tip of the electrode is closer to the base unit. For example, the farther the tip of the electrode is from the electrode clamping system of the electrochemical machining system, the more it may move laterally, thereby reducing dimensional control. Such lateral movement may occur when the electrolyte used in the ECM process flashes, causing the electrode 42 to move or vibrate.
The intermediate channel will be subsequently enlarged via ECM using a second, different electrode to form a final channel. The second electrode may be inserted into the component at the same location (e.g. same end of the pilot channel 40) as was the first electrode 42. Alternately, the second electrode may be inserted from another location on the component, for example, the opposite end of the pilot channel 40. The choice of using the same entry location or different entry locations applies to all channels formed via ECM using the first electrode 42 and the second electrode. In general, the final channel may alternately be formed from the pilot channel 40 in one electrochemical machining (enlarging) operation, or it may take several different electrochemical machining operations, and the electrode used during each operation may be the same or different than electrodes using during the other operations. The electrodes may be different in cross sectional shape, diameter, length, twist, and amount and pattern of the masking, etc. as necessary to reach the desired final contour. In addition, the electrodes may remove material in front of a tip of an advancing electrode and/or astride sides of the electrode when the electrode is advancing or when it is stationary.
As shown in
Once electrochemical machining commences in
As the first electrode 42 sinks into the airfoil 14 a thickness of the airfoil 14 increases. To maintain a desired pressure side intermediate wall thickness 60 and/or suction side intermediate wall thickness 62, a feed rate at which the first electrode 42 is lowered into the airfoil 14 may be controlled. In conventional electrochemical machining, the feed rate is typically selected to match a steady-state rate of liquefaction of the material being machined (material liquefaction rate) that occurs under the given set of electrochemical machining conditions, including, for example, the amount of current being used etc. This is done because the electrode is advancing toward the material being machined. This provides the fastest possible machining of the component. However, the process disclosed herein also machines material positioned on the sides of the electrode, as opposed to only in front of an advancing electrode. Accordingly, the electrode feed rate may be slowed to a rate below the material liquefaction rate. The slower electrode feed rate slows the passing of the electrode past the material astride the electrode, and this allows for the removal of more material astride the electrode than would occur of the electrode feed rate equaled the material liquefaction rate.
Stated another way, the first electrode 42 may produce a channel having more than one diameter simply by changing the electrode feed rate. In exemplary embodiments where there is no material in front of the first electrode 42, the electrode feed rate may even be greater than the material liquefaction rate if it is desired to minimize machining of material astride the first electrode 42.
For example, to maintain a specified wall thickness the feed rate the first electrode 42 is lowered may be slowed to below the liquefaction rate as the tip of the first electrode 42 approaches the base of the airfoil 14. The amount of material removed increases as the first electrode 42 is slowed. Thus, a diameter of the intermediate channel will increase with the increasing (overall) thickness of the airfoil 14 in order to ensure the intermediate wall thicknesses are controlled as desired.
In an exemplary embodiment, the intermediate wall thicknesses may remain constant from the tip 20 of the airfoil 14 to the base. In this case the intermediate wall thicknesses are directly responsive to an outer contour 64 of the airfoil 14 from the tip 20 to the base, where the outer contour 64 is defined by an outer surface 66 of the airfoil 14. If the intermediate wall thickness is constant at any given radial cross section then the intermediate wall thickness is directly responsive to the outer contour 64 at the given radial location. Alternately, the intermediate wall thicknesses may increase toward the base, but the diameter of the intermediate channel contour 44 may also increase. In this case the wall thickness is different for two different radial cross sections, but within each radial cross section the thickness is the same. In this case the intermediate wall thickness is indirectly responsive to the outer contour 64 from the tip 20 to the base. So long as a relationship between the outer contour 64 and the intermediate wall thickness exists, then the intermediate wall thickness is responsive to the outer contour 64 of the airfoil 14. The same principles apply to a final wall thickness of the airfoil.
Similarly, if the intermediate channel contour or the final channel contour (e.g. the cooling channel) follows a twist of the airfoil 14, for example, then the intermediate channel and/or the final channel are considered responsive to the outer contour 64 from the tip 20 to the base. Likewise, if at a given radial cross section a contour of the intermediate channel or of the final channel follows a respective portion of the outer contour 64, for example, then the intermediate channel and/or the final channel are considered responsive to the outer contour 64 at the radial location. So long as a relationship can be found between the outer contour 64 and the intermediate channel contour or the final channel contour, then the intermediate channel and/or the final channel are considered responsive to the outer contour 64.
Alternately, or in addition to varying the electrode feed rate to maintain a specified wall thickness, the amount of current through the first electrode 42 may be increased as the tip of the first electrode 42 approaches the base of the airfoil 14. The amount of material removed increases as the current increases. Thus, a diameter of the intermediate channel will increase in order to ensure the intermediate wall thicknesses are controlled as desired. The wall thicknesses and the channel diameter may be controlled by controlling the current to achieve the same results described above when the electrode feed rate is controlled.
The electrode feed rate and the current may be controlled independently or simultaneously as desired and may increase and/or decrease toward the base as necessary to achieve the desired profiles.
Alternately, or in addition, more than one ECM electrode may be used to form a single channel, for example, the pilot channel 40, the intermediate channel 90, or the final channel. In an example embodiment where the pilot channel 40 is formed via electrochemical machining, instead of using one ECM electrode to form the entire length of the pilot channel 40 in one operation, two ECM electrodes could be used. An ECM electrode could be inserted into the component from a first location and a complementary ECM electrode could be inserted into the component from a second, different location. Both ECM electrodes could electrochemically machine a respective portion of the pilot channel 40, and both portions together would form the completed pilot channel 40. Alternately, one ECM electrode could be used to in two operations, each operation forming a respective portion of the pilot channel 40.
The respective electrochemical machining operations could occur sequentially or simultaneously. In an exemplary embodiment with sequential machining, the ECM electrode could be inserted at the tip 20 of the airfoil (or other component) and progress toward the platform 12/base to machine a portion of the pilot channel 40, while the complementary ECM electrode could then be inserted at the base and progress toward the tip 20 to form a remainder of the pilot channel 40. In another exemplary embodiment the ECM electrode could be inserted at one location to form a portion of the pilot channel 40. After the first electrochemical machining operation completes the respective portion of the pilot channel 40, the ECM electrode could be retracted, the component repositioned, and the same ECM electrode may be inserted at another location to finish machining the remainder of the pilot channel 40. The operation that occurs second in time would simply stop upon reaching the portion of the pilot channel 40 formed by the operation that occurred first in time.
If the operations occur simultaneously, the ECM electrode could be inserted at the tip 20 of the airfoil (or other component) and progress toward the platform 12/base to machine a portion of the pilot channel 40, while the complementary ECM electrode is simultaneously inserted at the base to form a remainder of the pilot channel 40. The two ECM electrodes progress toward each other until they meet. Alternately, the operations could be a combination of sequential and simultaneous. For example, at one point in the operation both electrodes could be machining simultaneously until they get close to each other, at which point one electrode stops electrochemically machining, leaving the remainder of the pilot channel 40 to be electromechanically machined by the electrode that remains operational. Any combination of sequential and simultaneous may be applied.
Alternately, or in addition, the first electrode 42 may be masked to achieve the desired shape. This may occur when, for example, the cross sectional shape of the electrode is not the same as the desired contour. For example, if the first electrode 42 instead had a circular cross section, masking material could be selectively disposed on a first electrode pressure side 70 and on a first electrode suction side 72 but not on a first electrode leading edge 74 or on a first electrode trailing edge 76. (I.e. the first electrode 42 may be partly isolated.) In such an exemplary embodiment the masking would prevent machining (material removal) adjacent the first electrode pressure side 70 and the first electrode suction side 72, but would permit material removal adjacent the first electrode leading edge 74 and the first electrode trailing edge 76. Thus, when masked, it is possible for a first electrode 42 with a circular cross section to form a channel with an oval profile, and the diameter of the oval channel may be controlled by controlling the electrode feed rate and/or the current through the electrode. Any or all of these techniques may be employed as desired to achieve the desired contour and dimensions of the channel and wall and/or of a rib.
Also visible in
In the exemplary embodiment of
In addition, in this exemplary embodiment a centroid 96 of the second electrode 92 may or may not coincide with a centroid 98 of the intermediate channel 90 at any or all points from the tip 20 to the base. This may occur when a centroid 100 of the final contour 94 also does not coincide with the intermediate channel centroid 98. This lack of coincidence may be a result of, for example, a skewed corner 102 of the final contour 94, or other similar feature of the final contour 94. Alternately, the pilot channel 40 and the intermediate channel 90 may be positioned so their respective centroids all coincide.
As a result of the eccentricity of the second electrode 92 within the intermediate channel 90, if the second electrode 92 is not masked, material may not be removed uniformly around the second electrode 92. For example, material adjacent a second electrode leading edge 110 and a second electrode pressure side 112 may be removed faster than material adjacent a second electrode suction side 114 and a second electrode trailing edge 116. Should this effect cause the final contour 94 to be reached sooner on the second electrode suction side 114 and the second electrode trailing edge 116, the second electrode centroid 96 can be repositioned within the cross section as necessary before and/or during the electrochemical machining operation. Similarly, the centroid for any electrode may be repositioned during the electrochemical machining operation to account for any expected or unexpected material removal rates to ensure that the desired contours are formed.
The second electrode 92 may be lowered into the airfoil 14 and blade 10 until the cooling channel is formed having the final contour 94. As a result an airfoil external wall 120 is formed, which may be considered to include a pressure side wall 122 having a pressure side wall thickness 124 and a suction side wall 126 having a suction side wall thickness 128. In an embodiment, once formed, the leading edge cooling channel (represented by the leading-edge cooling channel contour 80), the middle cooling channel (represented by the final contour 94), and the trailing edge cooling channel (represented by the trailing edge cooling channel contour 82) form respective portions of the pressure side wall 122 and the suction side wall 126, both of which are thin walls, thereby forming a component having a thin-walled airfoil. Between the leading edge cooling channel and the middle cooling channel a first rib 130 is formed, and between the middle cooling channel and the trailing edge cooling channel a second rib 132 is formed. A thickness 134 of the first rib 130 and a thickness 136 of the second rib 132 may also be controlled via electrochemical machining using the methods disclosed above.
Also visible in
The masking blocks the electrochemical machining such that electrochemical machining occurs only adjacent the exposed pressure side surface 148 and the exposed suction side surface (not shown). The flow-interrupting elements 140 are thus raised portions of the pressure side wall 122 and the suction side wall 126 between areas that were electrochemically machined by the exposed pressure side surface 148 of the feature-forming electrode 142 and the exposed suction side surface (not shown) of the feature-forming electrode 142. Alternately, or in addition, the masking 144 may be reversed and the flow-interrupting elements 140 may be electrochemically machined/recessed into the pressure side wall 122 and/or the suction side wall 126.
Once the machining (material removal) is complete, in certain exemplary embodiments the tip 20 of the airfoil 170 may be capped. This may occur in any way known to those in the art, including welding a tip cap (e.g. a metal plate, not shown) into place. Once the tip cap is secured, the blade is complete and may include a serpentine cooling channel. In an exemplary embodiment a relatively small hole may be formed at the tip for each cooling channel via electrochemical machining, the cooling channel may be formed with the diameter increased relative to the small hole using the methods disclosed above. Once the electrode is removed the small hole formed in the tip 20 may simply be welded closed. Alternately, the tip 20 may not be partly capped, or not capped at all.
The electrode may be lowered into the blade in other directions as necessary. For example, the electrode may be angled forward, backward, or to a side to form some of the more complex geometry. It is also possible that the electrode may be lowered into the blade 170 from the base 176 toward the tip 20 as necessary. Any and all of the above methods may be used as necessary to create the desired inner contours.
The inventors have proposed to use electrochemical machining in novel method of manufacturing a gas turbine engine component, such as a component including a thin-walled airfoil. This process dispenses with the need to have precise control of the formation of a core and/or the airfoil during an investment casting process, and in exemplary embodiments dispenses with the core entirely. This renders mood any associated tolerance control issues. In electrochemical machining the electrode does not contact the component or wear out when forming the component. Consequently, the electrode lasts longer than conventional machine tooling. Reduced heat and physical and thermal stresses occur during electrochemical machining, increasing part yield. As a result, components with tight tolerance control can be reliably and accurately formed with increased yield, and this may occur at a relatively lower cost. This is particularly true for high volume production operations, where the initial cost of the electrode is outweighed by the longer life. Consequently, the process represents an improvement in the art.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
This application is a U.S. National Stage Application of International Application No. PCT/US2015/043971, filed Aug. 6, 2015, which claimed priority to U.S. Provisional Patent Application No. 62/068,099, filed on Oct. 24, 2014. The entirety of each application is incorporated by reference herein in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2015/043971 | 8/6/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62068099 | Oct 2014 | US |
