Method for manufacturing an impeller and impeller

Abstract

The present invention relates to a method for manufacturing an impeller (210), in particular for a turbomachine, the impeller having a closed design, the impeller comprising a first portion (230), a second portion (232), and vanes (234) arranged between the first portion (230) and the second portion (232), forming channels (240) in the impeller (210), the method comprising the steps of: providing a green-stage impeller comprising the first portion (230) and the vanes (234) arranged on the first portion (230); providing a filler material in the spaces between the vanes (234); forming the second portion (232) on a top surface of the filler material and on a top side of the vane, preferably by cold spraying; and removing the filler material.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an impeller of closed design, for example for turbomachines, to such an impeller, and to a turbomachine comprising such an impeller. [Background technology]

[0002] Turbomachines such as compressors, expanders, and pumps can be used in a number of different applications. For example, cryogenic applications, i.e., applications using cryogenic process gases, such as air separation plants, often use cryogenic turbomachines such as turboexpanders and / or turbocompressors. Such turbomachines typically include an expander impeller and / or a compressor impeller fixed to a shaft.

[0003] Impellers of such turbomachines can have an open or closed design, with the impeller blades forming channels. The advantage of a closed design is that the impeller has a higher isentropic efficiency. Under certain processing conditions, i.e., often very low specific speeds and high rotational speeds, the ideal geometry of such channels, which would result in the highest efficiency, cannot be machined by conventional subtractive techniques. In these cases, the impeller must remain open (i.e., the impeller is of open design), with accompanying efficiency losses. It is therefore an object of the present invention to provide an improved closed-design impeller.

[0004] Disclosure of the Invention This object is achieved by providing a method for manufacturing an impeller, an impeller and a turbomachine with the features of the independent claims.Embodiments of the invention are the subject of the dependent claims and the following description.

[0005] The present invention relates to the manufacture of closed-design impellers for turbomachines such as expanders, compressors, or pumps, comprising a first portion (or shroud or plate, which may be a bottom portion), a second portion (or shroud or plate, which may be a top portion), and vanes arranged between the first and second portions to form a channel (for the working medium) within the impeller.

[0006] As mentioned above, the ideal geometry of such channels, which provides the highest efficiency, typically cannot be machined using conventional subtractive techniques. In these cases, the impeller must remain open, i.e., the impeller has an open design. Additive manufacturing techniques make it possible to build impellers with a closed design (closed impellers). However, typical additive manufacturing techniques involve intense heating of parts that are constructed entirely by additive manufacturing (Powder Bed Fusion-Selective Laser Melting (PBF-SLM)) or partially by adding functionality to wrought components (Direct Energy Deposition (DED) processes). Such intense heating is a problem when building downskin surfaces because the heat must be evacuated. Otherwise, the material will burn, forming ledges and dross. Therefore, supports can be used. However, supports within inaccessible channels are difficult to remove, and assuming successful removal, the remaining surface finish typically has a surface roughness Ra of greater than 10 μm. This is also true with small angle downskin surfaces, which can sometimes be constructed without supports. Upskin surfaces are also typically too rough.

[0007] Brazing can be used to join a milled, expanded, open impeller (e.g., the first part mentioned, with its blades) with a separate shroud (such as the second part mentioned), which is also turned (e.g., milled on a lathe). However, brazing is not applicable to aluminum alloys, such as the strongest alloys, and remains difficult with titanium alloys, as the brazed interface is highly prone to defects and remains a weak point.

[0008] For example, it has been found that using cold spray (or cold welding) techniques, for example, it is possible to obtain an additively manufactured shroud (second part) on an expanded open impeller (first part with blades) without intensely heating the part. Such a lack of heat allows for the use of filler materials, such as fusible materials, to connect the sprayed material. Furthermore, it has been found that no weak points at the interface between the two materials occur, since the interface shape can be customized as needed when using such a two-step approach. This allows for the manufacture of closed impellers in which, as built and prior to any subsequent optional surface treatments, the surface finish of the internal channels, i.e., the inner surfaces of the channels, exhibits a surface roughness Ra of approximately 1.6 μm and a surface roughness Rt of approximately 10 μm in the down skin and up skin.

[0009] Taking this into consideration, a method for manufacturing an impeller in multiple steps is proposed. In a first step, a green-stage impeller part is provided, the (green-stage) impeller comprising a first section and blades arranged in the first section. This first section can also be called a bottom section, since it is typically arranged on a lower working surface. In this green stage, the impeller has an open (or semi-open) design, since the first section (or top) is absent.

[0010] In one embodiment, the green stage impeller is provided as a wrought material part, for example, the green stage impeller is made from one of the following materials: aluminum, aluminum alloy, titanium, titanium alloy, nickel, nickel alloy, stainless steel.

[0011] At least a portion of the inner surface of the channel to be formed (i.e., the surface of the first portion located between the vane and the side of the vane) is machined to a final profile. Such a final profile may be the profile that the surface will have in the final impeller (preferably after some further surface treatment). The top side of the vane (i.e., the side of the vane where the second portion will be added) is pre-machined. The pre-machining does not result in a final profile, but some material remains to be removed by further machining. In addition, other remaining surfaces may be pre-machined.

[0012] In embodiments, the top sides of the vanes can have one of the following final profiles: a rectangular geometry, a flared geometry, a geometry with side gaps, or a geometry with full gaps. These geometries provide different bonding surfaces for a second portion to be added later. Further details of these geometries are described with respect to the drawings. Preferably, a geometry with side gaps or full gaps between the vanes has the advantage that during operation, no maximum stresses are located at the interface between the wrought material and the sprayed material.

[0013] In a further step, a filler material, preferably a soluble material, is provided in the spaces between the vanes. These spaces between the vanes (and the surfaces of the first portions located between the vanes) will become the inner surfaces of the channels (preferably after some further surface treatment). This filler material is preferably provided so that its top surface (i.e., the surface oriented toward the second portion of the impeller to be added) or at least part of this surface has the contour that this second portion will later have. In addition, the top side of the vane should have the final contour so that the second portion can then be added directly.

[0014] This is achieved by the following steps (which may also be declared as sub-steps): First, the spaces between the vanes are filled with a filler material so that it exceeds or at least reaches the top sides of the vanes. It should be noted that the top sides of the vanes are only pre-machined. Then, the top sides of the vanes are machined to their final profile, and the filler material is also machined to a profile intended to have the second portion at its top surface (or at least part of it).

[0015] The advantage of both the filler material and the substrate surface being machined in the same machining operation is to provide the desired fillet radius for the channel and sufficient contact area between the base material and the sprayed material.

[0016] Additionally, any remaining filler material (if present) can be removed from the top side of the vane to achieve the final profile, where etching can be used, for example.

[0017] In a further step, a second portion is formed on the top surface of the filler material and the top side of the vane, preferably by cold spraying. Cold spraying (CS) or gas-dynamic cold spraying is a coating deposition method. Solid powder (e.g., 1 to 50 micrometers in diameter) can be accelerated in a supersonic gas jet, for example, up to Mach 4. The particles undergo plastic deformation during impact with the substrate and adhere to the surface. To achieve a uniform thickness, the spray nozzle can be scanned along the substrate. The kinetic energy of the particles, provided by the gas expansion, is converted into plastic deformation energy during bonding. Unlike thermal spraying techniques, such as plasma spraying, arc spraying, flame spraying, or high velocity oxygen fuel (HVOF), the powder is not melted during the spraying process.

[0018] In one embodiment, the second part obtained by the cold spraying process is made of the same material as the green stage impeller or has a chemical composition suitable for a strong bond with the green stage impeller material, which ensures sufficient stability of the final impeller.

[0019] In an optional step, the impeller can be heat treated (before removing the filler material). In a further step, the filler material is removed, for example, by melting it. For example, if the substrate material (of the first part) is a titanium alloy, an aluminum alloy such as 7075 can be used. Such an aluminum alloy can be dissolved, for example, with NaOH. In an optional further step, after removing the filler material, the impeller can be heat treated (again). Such heat treatment can be useful to relieve residual stresses or to induce diffusion between the low-temperature spray material and the wrought material, increasing the interfacial resistance.

[0020] Preferably, in a further step, after the removal of the filler material, the inner surface of the channel can be post-treated, preferably by at least one of abrasive flow machining, hirtisation (hirtisation is based on a combination of electrochemical pulsing, hydrodynamic flow and particle-assisted chemical removal and surface treatment, the material-specific treatment media used result in a gentle surface finishing effect and there are no harsh mechanical treatment steps), micromachining, etc. In this way, the surface can be further improved.

[0021] The invention also relates to an impeller obtainable by the method described above, and to a turbomachine including such an impeller.

[0022] Further advantages and embodiments of the invention will become apparent from the description and accompanying drawings.

[0023] It should be noted that the features mentioned above and further described below can be used not only in the respective combinations shown, but also in further combinations or alone, without departing from the scope of the invention. For example, in a different approach, the step of pre-machining at least a portion of the top side of the vane or the inner surface of the channel to the final profile before providing the filler material could possibly be avoided. [Brief explanation of the drawings]

[0024] [Figure 1] 1 illustrates a turbomachine having an impeller according to one embodiment of the present invention; [Figure 2] 1 illustrates an impeller according to a further preferred embodiment of the present invention. [Figure 3] Illustrates an impeller with an open design. [Figure 4] A flow diagram illustrates a method according to a preferred embodiment of the present invention. [Figure 5] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 6a] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 6b] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 7] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 8] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 9a] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 9b] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 9c] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 10] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 11a] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 11b] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; [Figure 11c] 1 illustrates a portion of an impeller according to a further embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0025] FIG. 1 schematically illustrates a turbomachine 100 according to a preferred embodiment of the present invention. The turbomachine 100, e.g., a cryogenic turbomachine, is configured, by way of example, as a compressor and an expander, i.e., both are combined in one turbomachine. Accordingly, the turbomachine 100 comprises two impellers, impeller 110 and impeller 120, both of which are attached to a shaft 130. The turbomachine 100 comprises channels 112 and 114 on the side of the impeller 110, which serve as inlet and outlet channels, respectively, for a working medium or working fluid (see arrow 113) that is compressed and subsequently withdrawn (see arrow 115). The turbomachine 100 further comprises channels 122 and 124 on the side of the impeller 120, which serve as inlet and outlet channels, respectively, for a working fluid to be expanded. Accordingly, the impeller 110 is a compressor impeller, and the impeller 120 is an expander impeller.

[0026] FIG. 2 schematically illustrates an impeller 210 according to a further preferred embodiment of the present invention. While the impellers 110, 120 of FIG. 1 are shown schematically, the impeller 210 is shown as a closed impeller, i.e., an impeller of closed design. The impeller 210 comprises a plurality of vanes 234 surrounded by two shrouds or plates 230 and 232. A channel 240 is thus formed between two of these vanes and the plate. The impeller 210 comprises a rotation axis 250. As can be gathered from FIG. 2, machining of such a channel 240, particularly in the inner region, can be complicated or even impossible.

[0027] For purposes of explanation, Figure 3 illustrates schematically an impeller 310 similar to impeller 210, but with an open (or semi-open) design. Impeller 310 comprises a number of vanes 334 arranged on shroud or plate 230. In this way, a kind of (open) channel 340 is formed between two of these vanes and the plate. As can be deduced from Figure 3, machining of such a channel 340 is much better feasible than for channel 240 of Figure 2.

[0028] FIG. 4 illustrates, by way of a flow diagram, a method according to a preferred embodiment of the present invention. The method includes several different steps. In step 400, a green-stage impeller part is provided. The impeller (green stage) comprises a first part 230 (see FIG. 2) and blades 234 arranged on the first part 230. In this green stage, the impeller has an open (or semi-open) design, since the first (or top) part 232 is not yet present. In this green stage, the impeller looks like the impeller 310 shown in FIG. 3. In one embodiment, the green-stage impeller is provided as a wrought material part. For example, the green-stage impeller is made of one of the following materials: aluminum, aluminum alloy, titanium, titanium alloy, nickel, nickel alloy, stainless steel.

[0029] In step 402, at least a portion of the inner surface of the channel 240 to be formed is machined to the final profile. In step 404, the top sides of the vanes 234 are pre-machined. Thus, for example, an impeller is machined to its final profile in the channel, but is only pre-machined elsewhere. The top sides or surfaces of the vanes (or blades) are also only pre-machined. The pre-machining does not result in the final profile, but some material remains to be removed by further machining. Milling allowance should still exist.

[0030] This is illustrated in more detail in Figure 5, which shows a cross-section (shaded area) of first portion 530 with vanes 534 and rotation axis 550 (similar to first portion 230 and rotation axis 250 in Figure 2). In addition, top sides 536 and surfaces 538 of the vanes are shown. Surface 538 comprises the sides of vanes 534 and the top surface of first portion 530, which will later become the inner surfaces of the channels. This surface 538 will be machined to its final profile. Only top sides 536 or surfaces and other surfaces of the vanes are pre-machined.

[0031] The top side of the vane cross section can be, for example, rectangular as illustrated in FIG. 6a, or can be a flared geometric shape (with a flare) as illustrated in FIG. 6b. FIGS. 6a and 6b show cross-sectional views of the first portion 630 and vanes 634a, 634b (e.g., as viewed from the left in FIG. 5). The top side 636a is a rectangular shape or geometric shape. For example, the top side 636b includes a flare with a chamfer of length d, extending at angle a by radius R, provided that it is machinable by five-axis milling. The choice of vane top profile can depend on the interface resistance required between the first portion with the vane (wrought material) and the cold-sprayed second portion (shroud).

[0032] In step 406, a filler material, preferably a soluble material, is provided in the spaces between the vanes. These spaces between the vanes (and the surfaces of the first portions whose surfaces are located between the vanes) become the inner surfaces of the channels (preferably after some further surface treatment).

[0033] Step 406 includes step 408, which includes filling the spaces between the vanes with filler material such that the filler material extends beyond or at least reaches the top sides of the vanes. The filler material may be applied by cold thermal spraying.

[0034] This is illustrated in more detail in Figure 7. Figure 7 corresponds to Figure 5, but additional filler material 740 is provided between vanes 534 and therefore also on surfaces 538. It can be seen that filler material 740 extends beyond top sides 536 of vanes 534.

[0035] Step 406 further includes step 410, in which the top side of the vane is machined to its final profile. Step 406 also includes step 412, in which the filler material is machined on its top surface (or at least a portion thereof) to the profile that the second portion is intended to have. This is illustrated in more detail in FIG. 8. FIG. 8 corresponds to FIG. 7, but the top side 536 now has its final profile, illustrated in bold. The filler material 740 is still present. In an additional step 414, remaining filler material (if present) can be removed from the top side of the vane to achieve the final profile, which can be achieved using, for example, etching. The top side of the vane is now free of filler material.

[0036] The machining of the top side in step 410 can be done in different ways, some of which are illustrated in Figures 9a, 9b, and 9c. Figure 9a corresponds to Figure 6a, but in which filler material 740 is provided and the top side of the vane is rectangular. Then, for example, machining of the filler (soluble) material can be done only on the top of the vane.

[0037] Figure 9b corresponds to Figure 6b, but the top sides of the vanes are flared, provided with filler material 740. Machining of the filler (soluble) material can then be performed, for example, only on the tops of the vanes.

[0038] FIG. 9c shows a similar view, but with filler material 740 provided, and the top side of the vane being rectangular. Then, for example, filler (fusible) material can be machined onto the top of the vane and even onto the tops of the sides of the vane. This results in the final profile of the top side 936c with side gaps. The shape of the side machining can depend on the interface resistance required between the wrought material (first portion and vane) and the cold-sprayed material. In particular, the parameters of the gap length g, gap depth h, radii r, t, and b are determined according to the strength required at the interface. For example, the length g must be wide enough, and the depth h should not be so deep as to allow the cold-sprayed material to fill the entire gap. In another preferred embodiment (not shown), g can be as wide as the opposite wall, so there would be one large gap instead of two side gaps.

[0039] The gap geometry is not limited to the geometry shown in FIG. 9c. For example, any gap geometry that can be machined by 5-axis milling can be considered. The gap in FIG. 9c is depicted as a rectangle, but it could also be triangular, for example. A consideration for defining the gap geometry is that machining should be limited primarily to the wrought material to avoid clogging the channels after the shroud is constructed. The only acceptable machining of the filler material is a radius r comparable to the radius between the blade leg and the impeller hub.

[0040] In step 416, a second portion (shroud) is formed on the top surface of the filler material and the top side of the vane, preferably by cold spraying, of a material or alloy that is preferably chemically identical or compatible for strong bonding with the wrought material.

[0041] This is illustrated in more detail in Figure 10, which corresponds to Figure 8, but provides a second portion 1032 (which may correspond to second portion 232; see Figure 2). The cold spray process is indicated by a spray nozzle or gun 1042. As illustrated, the gun 1042 can be oriented perpendicular to the surface of the top side 536 to achieve good results. Experiments have shown that spraying perpendicular to the surface provides the best surface finish of the sprayed layer after melting of the filler material.

[0042] This is also illustrated in Figures 11a, 11b, and 11c, which correspond to Figures 9a, 9b, and 9c, respectively. In addition, however, a second portion 1032 (see Figure 10) is also provided. In particular, Figure 11c shows that the second portion is also present in the gap.

[0043] In optional step 418, the impeller can be heat treated. In step 420, the filler material is removed, for example by melting it. In optional step 422, the impeller can be (re)heated. In step 424, the inner surface of the channel 240 (see FIG. 2) can be post-treated, preferably by abrasive flow machining, hirtisation or micro-machining. In this way, the surface can be further improved. In step 426, the outer geometry of the impeller can be finished by turning and milling.

Claims

1. A method for manufacturing an impeller (110, 120, 210) for a turbomachinery (100), wherein the impeller has a closed design, and the impeller comprises a first portion (230, 530), a second portion (232, 1032), and blades (234, 534, 634a, 634b) positioned between the first portion (230, 530) and the second portion (232, 1032), and a channel (240) is formed within the impeller (210), and the method is - A step (400) of providing the impeller in an untreated state, comprising the first portion (230, 530) and the blades (234, 534) arranged in the first portion (230, 530), - A step (406) of providing a filling material (740) in the space between the blades (234, 534), - A step (416) to form the second portion (232, 1032) on the top surface of the filler material (740) and the top side portion (536, 636a, 636b, 936c) of the blade by cold spraying, - Includes a step (420) of removing the filling material (740), Before providing the aforementioned filling material, - A step (402) of machining at least a portion of the inner surface of the channel to be formed into a final contour, - A step (404) of pre-machining the top portion of the blade, The step (406) of providing the filling material to the space between the blades is, - A step (408) of filling the space between the blades with the filling material by cold spraying such that the filling material reaches at least the top portion of the blade or exceeds the top portion, - A step (410) of machining the top portion of the feather to its final contour, A method comprising the step (412) of machining at least a portion of the top surface of the filler material into the contour of the second portion that is intended to have.

2. The method according to claim 1, wherein the step of providing the filler further includes a step (414) of removing the remaining filler from the apex of the blade, preferably by etching, after machining the apex of the blade, in order to achieve the final contour.

3. The method according to claim 1 or 2, further comprising the step (418, 422) of heat-treating the impeller before and / or after removing the filler material.

4. The method according to claim 1 or 2, further comprising a step (424) of post-treating the inner surface of the channel, preferably including at least one of abrasive flow machining, hirtisation, and micro-machining, after removing the filler material.

5. The method according to claim 1 or 2, wherein the filling material is a soluble material, and the step of removing the filling material includes a step of melting the filling material.

6. The method according to claim 1 or 2, wherein the impeller in the untreated stage is provided as a wrought material component.

7. The method according to claim 1 or 2, wherein the impeller in the untreated stage is made of one of aluminum, aluminum alloy, titanium, titanium alloy, nickel, nickel alloy, and stainless steel.

8. The method according to claim 1 or 2, wherein the second portion (232, 1032) obtained by cold spraying is made of the same material as the untreated impeller, or has a chemical composition suitable for a strong bond with the material of the untreated impeller.

9. The method according to claim 1 or 2, wherein the cold spraying is performed using a spray orientation perpendicular to each surface.

10. The method according to claim 1 or 2, wherein the top portion (636a, 636b, 936c) of the blade (234, 536, 636a, 636b) has one of the following final contours: a rectangular geometric shape, a flared geometric shape, a geometric shape with lateral gaps, or a geometric shape with a complete gap.

11. Preferably, an impeller (110, 120, 210) for a turbomachinery (100), having a closed design, comprising a first portion, a second portion, and blades positioned between the first portion and the second portion, wherein channels (240, 340, 540) are formed within the impeller, and the impeller is obtained by the method of claim 1 or 2.

12. A turbomachinery (100) comprising the impellers (110, 120, 210) according to claim 11.

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