Method for manufacturing impeller and impeller

Abstract

The invention relates to a method for manufacturing an impeller (210), in particular for a turbomachine, said impeller having a closed design and comprising a first part (230), a second part (232) and blades (234) arranged between the first part (230) and the second part (232) so as to form channels (240) in the impeller (210), the method comprising the steps of: providing a primary stage of the impeller, comprising a first part (230) and a blade (234) arranged at the first part (230); a filler material provided in a space between the blades (234); forming a second portion (232) at the top surface of the filler material and at the top side of the blade by cold spraying; and removing the filling material.

Description

[0001] The invention relates to a method for producing an impeller of closed design, for example for a turbomachine, an impeller of such a type and a turbomachine having such an impeller. Background Art

[0002] Turbines such as compressors, expanders and pumps can be used in different applications. For example, in cryogenic applications, i.e., applications where the process gas is at cryogenic temperatures, such as for air separation and other facilities, cryogenic turbines such as turboexpanders and / or compressors are often used. Such turbines usually include an expander impeller and / or a compressor impeller fixed to a shaft.

[0003] The impeller for such a turbine can be of open design or of closed design, in which the blades of the impeller form channels. The advantage of the closed design is a higher isentropic efficiency of the impeller. Under certain process conditions, often at very low specific speeds and high rotational speeds, the ideal geometry of such channels leading to optimal efficiency cannot be machined by conventional subtractive techniques. In these cases, the impeller must remain open (i.e. the impeller is in open design), which involves a loss of efficiency. It is therefore an object of the present invention to provide an improved impeller of closed design. Summary of the invention

[0004] This object is achieved by providing a method for producing an impeller, an impeller and a turbine having 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 an impeller of closed design for use in a turbomachine such as an expander, a compressor or a pump. Such an impeller comprises a first part (or a shroud or a plate; it can be a bottom part), a second part (or a shroud or a plate; it can be a top part) and blades arranged between the first part and the second part, thereby forming a channel (for the working medium) in the impeller.

[0006] As mentioned above, the ideal geometry of such channels that lead to optimal efficiency cannot usually be machined by traditional subtractive techniques. In these cases, the impeller must remain open, that is, the impeller is in an open design. Additive manufacturing technology makes it possible to build impellers with closed designs (closed impellers). However, typical additive manufacturing techniques involve strong heating of the part, which is either built entirely by additive manufacturing (PBF-SLM, powder bed fusion, selective laser melting) or partially by adding functions to forged parts (DED, direct energy deposition, process). Because the heat must be discharged, such strong heating is a problem for building the lower skin surface, otherwise the material burns and forms protrusions and scum. Therefore, supports can be used; however, it is difficult to remove supports in inaccessible channels, and assuming successful removal, the surface finish left behind has a surface roughness Ra of typically above 10μm. The same is true for low-angle lower skin surfaces, which can sometimes be built without supports. The upper skin surface is also usually too rough.

[0007] Brazing can be used to bond forged open impellers (e.g., the first part mentioned with blades) that have been milled together with a separate shroud (e.g., the second part mentioned) that has also been turned (e.g., milled on a lathe). However, brazing is not suitable for aluminum alloys, such as the strongest aluminum alloys, because the brazing interface is very prone to defects and remains a weak point, and brazing remains difficult to achieve for titanium alloys.

[0008] It turns out that the cold spraying (or cold welding) technique can be used, for example, to obtain an additively manufactured shroud (second part) on, for example, a forged open impeller (first part with blades), without involving intensive heating of the parts. Such lack of heat makes it possible to use filler materials such as soluble materials to fix the sprayed material. In addition, it turns out that there are no weak points at the interface between the two materials, because when adopting such a two-part approach, the interface shape can be customized according to the requirements. This makes it possible to manufacture a closed impeller with a surface finish of the internal channels (i.e., the inner surface of the channels) at the lower skin and the upper skin, with a surface roughness Ra of the order of 1.6 μm and a surface roughness Rt of the order of 10 μm when built and before any subsequent optional surface treatment.

[0009] Therefore, a multi-step method is proposed for manufacturing an impeller. In a first step, a preliminary stage of an impeller part is provided, the impeller (in the preliminary stage) comprising a first part and blades arranged at the first part. Since this first part is usually arranged at the working surface, wherein this part faces downwards, this first part may also be referred to as a bottom part. In this preliminary stage, since there is no first (or top) part, the impeller has an open (or semi-open) design.

[0010] In one embodiment, the original stage of the impeller is provided as a forged material part. For example, the original stage of the impeller is made of 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 part between the blades, and the side surface of the blades) is machined to a final profile. Such a final profile may be the profile that the surface should have in the final impeller (preferably after some additional surface treatment). And the top side of the blade (i.e., the side of the blade to which the second part is to be added) is pre-machined. Pre-machining does not produce a final profile, but still leaves some material that needs to be removed by further machining. In addition, the other remaining surfaces may be pre-machined.

[0012] In an embodiment, the top side of the blades may have one of the following final profiles: a rectangular geometry, a trumpet geometry, a geometry with side gaps or a full gap. These geometries will result in a second part to be added later having a different adhesion surface. More details of these geometries will be explained with reference to the accompanying drawings. Preferably, the advantage of the geometry with side gaps or a full gap between the blades is that the maximum stress in operation is not at all located in the interface between the forged material and the sprayed material.

[0013] In a further step, a filling material, preferably a soluble material, is provided in the spaces between the blades. These spaces between the blades (and the surfaces of the first part located between the blades) will become the inner surface of the channel (preferably after some further surface treatment). The filling material is preferably arranged so that the top surface of the filling material (i.e. the surface oriented towards the second part of the impeller to be added) or at least a part of this surface has the profile that the second part should have later. Furthermore, the top side of the blade should then have the final profile so that the second part can be added directly.

[0014] This will be achieved by the following steps (also referred to as sub-steps): First, the space between the blades will be filled with filler material so that the filler material exceeds the top side of the blade, or at least reaches the top side. It should be noted that the top side of the blade is only pre-machined. Then, the top side of the blade will be machined to the final profile, and the filler material at the top surface (or at least a portion thereof) will also be machined to the profile of the intended second part.

[0015] The advantage of machining both the filler material and the base material surface in the same machining operation is that the channels have the desired fillet radius and sufficient contact area between the base material and the sprayed material.

[0016] Furthermore, remaining filler material in the top side of the blade, if present, may be removed to achieve the final profile; for example, etching may be used here.

[0017] In a further step, a second portion is preferably formed at the top surface of the filler material and at the top side of the blade by cold spraying. Cold spray (CS) or gas dynamic cold spraying is a coating deposition method. Solid powder (e.g., 1 micron to 50 microns in diameter) can be accelerated to a speed of up to, for example, Mach 4 in a supersonic gas jet. During collision with the substrate, the particles are plastically deformed and adhere to the surface. In order to achieve uniform thickness, the nozzle can be scanned along the substrate. The kinetic energy of the particles supplied by gas expansion is converted into plastic deformation energy during bonding. Unlike thermal spraying techniques such as plasma spraying, arc spraying, flame spraying or high-speed oxygen fuel (HVOF), the powder does not melt during the spraying process.

[0018] In one embodiment, the second part obtained by cold spraying in a step is made of the same material as the original stage of the impeller or has a chemical composition compatible with strong bonding with the material of the original stage of the impeller. This ensures that the final impeller has sufficient stability.

[0019] In an optional step, the impeller may be heat treated (before the filling material is removed). In a further step, the filling material is removed, for example by dissolving it. For example, when the substrate material (for the first part) is a titanium alloy, an aluminum alloy such as 7075 may be used. Such an aluminum alloy may be dissolved, for example, with NaOH. In an optional further step, the impeller may be heat treated (again) after the filling material is removed. Such a heat treatment may be used to relieve residual stresses or to create diffusion between the cold sprayed material and the forged material to increase the interface resistance.

[0020] Preferably, in a further step, after removal of the filling material, a post-treatment may be performed on the inner surface of the channel, preferably comprising at least one of the following: abrasive flow machining, hirtisation (hirtisation is based on a combination of electrochemical pulse methods, fluid dynamic flow and particle-assisted chemical removal and surface treatment; the treatment medium of the specific material used produces a gentle surface finish effect; there are no harsh mechanical treatment steps), micromachining. In this way, the surface can be further improved.

[0021] The invention also relates to an impeller obtained by a method as described above, and to a turbomachine comprising such an impeller.

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

[0023] It should be noted that the features mentioned above and those to be described further below can be used not only in the respectively indicated combination but also in further combination or alone without departing from the scope of the present invention. For example, in a different manner, it may be possible to avoid the step of pre-machining at least a portion of the top side of the blade or the inner surface of the channel to the final contour before providing the filling material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A turbine with an impeller according to one embodiment of the invention is shown.

[0025] Figure 2 An impeller according to another preferred embodiment of the present invention is shown.

[0026] Figure 3 An impeller of open design is shown.

[0027] Figure 4 The method according to a preferred embodiment of the invention is shown in the flow chart.

[0028] Figure 5 A portion of an impeller according to another embodiment of the present invention is shown.

[0029] Figure 6a and Figure 6b Parts of an impeller according to another embodiment of the invention are shown.

[0030] Figure 7 A portion of an impeller according to another embodiment of the present invention is shown.

[0031] Figure 8 A portion of an impeller according to another embodiment of the present invention is shown.

[0032] Figure 9a , Figure 9b and Fig.9c Parts of an impeller according to another embodiment of the invention are shown.

[0033] Fig.10 A portion of an impeller according to another embodiment of the present invention is shown.

[0034] Fig.11a , Fig.11b and Fig.11c Parts of an impeller according to another embodiment of the invention are shown. DETAILED DESCRIPTION

[0035] Figure 1 A turbine 100 according to a preferred embodiment of the present invention is schematically shown. The turbine 100 (e.g., a cryogenic turbine) is, for example, constructed as a compressor and an expander, i.e., both are combined in one turbine. Therefore, the turbine 100 includes two impellers, an impeller 110 and an impeller 120, both of which are mounted on a shaft 130. The turbine 100 includes channels 112 and 114 located on the side of the impeller 110, which are used as inlet channels and outlet channels for a working medium or fluid (see arrow 113) to be compressed and then discharged (see arrow 115). The turbine 100 also includes channels 122 and 124 located on the side of the impeller 120, which are used as inlet channels and outlet channels for a working fluid to be expanded. Therefore, the impeller 110 is a compressor impeller, and the impeller 120 is an expander impeller.

[0036] Figure 2 An impeller 210 according to another preferred embodiment of the present invention is schematically shown. Figure 1 The impellers 110, 120 in the embodiment are schematically shown, while the impeller 210 is shown as a closed impeller, i.e., an impeller in a closed design. The impeller 210 includes a plurality of blades 234, which are surrounded by two shrouds or plates 230 and 232. Thus, a channel 240 is formed between two such blades and plates. The impeller 210 includes an axis of rotation 250. As can be seen from Figure 2 It follows that machining of such a channel 240, particularly in the inner region, may be complicated or impossible.

[0037] For illustrative purposes, Figure 3 Schematically shown is an impeller 310, which is similar to the impeller 210, but has an open (or semi-open) design. The impeller 310 comprises a plurality of blades 334, which are arranged on the shroud or plate 230. In this way, a certain (open) channel 340 is formed between two such blades and the plate. Figure 3 It can be inferred that the feasibility of machining such a channel 340 is better than that of machining a channel 340. Figure 2 The passage 240 is machined.

[0038] Figure 4 The method according to a preferred embodiment of the present invention is shown in the flowchart with the aid of a flow chart. The method comprises different steps. In step 400, an initial stage of an impeller section is provided. The impeller (in the initial stage) comprises a first section 230 (see Figure 2 ) and blades 234 arranged at the first portion 230. In this original stage, since there is no first (or top) portion 232 yet, the impeller has an open (or semi-open) design. In this original stage, the impeller looks like Figure 3The impeller 310 is shown. In one embodiment, the original stage of the impeller is provided as a forged material part. For example, the original stage of the impeller is made of one of the following materials: aluminum, aluminum alloy, titanium, titanium alloy, nickel, nickel alloy, stainless steel.

[0039] 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 side of the blade 234 is pre-machined. Thus, in the channel, the impeller is machined to its final profile, but it is only pre-machined in other locations, for example. The top side or surface of the blade (or blade) is also only pre-machined. Pre-machining does not produce the final profile, but still leaves some material that needs to be removed by further machining. Milling allowances should be retained.

[0040] This is shown in more detail in Figure 5 Here, a blade 534 and a rotation axis 550 (similar to Figure 2 230 and the axis of rotation 250). In addition, the top side 536 and the surface 538 of the blade are shown. The surface 538 includes the side surface of the blade 534 and the top surface of the first part 530, which will later become the inner surface of the channel. This surface 538 is machined to its final contour. Only the top side 536 or surface of the blade and other surfaces are pre-machined.

[0041] For example, the top side of the blade cross section may be as follows Figure 6a The rectangle shown or the Figure 6b The flared geometry shown (flared). Figure 6a and Figure 6b A cross-sectional view (eg, from the perspective of FIG. 1 ) of the first portion 630 and blades 634a, 634b is shown. Figure 5 636a is rectangular in shape or in a rectangular geometry. Top side 636b includes a flare having a chamfer of length d and angle α that extends radius R, provided that it can be machined by, for example, five-axis milling. The blade top profile can be selected based on the desired interface resistance between the first portion with the blade (forged material) and the cold-sprayed second portion (shroud).

[0042] In step 406, a filler material, preferably a soluble material, is provided in the spaces between the blades. These spaces between the blades (and the surfaces of the first portion located between the blades) will become the inner surfaces of the channels (preferably after some additional surface treatment).

[0043] Step 406 includes step 408, in which the space between the blades is filled with a filling material such that the filling material exceeds the top side of the blades, or at least reaches the top side. The filling material may be provided by cold spraying.

[0044] This is shown in more detail in Figure 7 middle. Figure 7 Corresponds to Figure 5 However, additional filler material 740 is provided between the vanes 534 and, therefore, also on the surface 538. It can be seen that the filler material 740 extends beyond the top side 536 of the vanes 534.

[0045] Step 406 also includes step 410, in which the top side of the blade is machined to the final profile. Step 406 also includes step 412, in which the filler material at its top surface (or at least a portion thereof) is machined to the profile that the second portion is intended to have. This is shown in more detail in Figure 8 middle. Figure 8 Corresponds to Figure 7 , however, the top side 536, shown here in bold, has its final contour. Filler material 740 is still present. In an additional step 414, the remaining filler material in the top side of the blade (if present) may be removed to achieve the final contour; for example, etching may be used here. The top side of the blade should be completely free of filler material.

[0046] In step 410, machining of the top side may be performed in different ways, some of which are shown in FIG. Figure 9a , Figure 9b and Fig.9c middle. Figure 9a Corresponds to Figure 6a , wherein a filling material 740 is provided; the top side of the blade is rectangular. Then, for example, machining of the filling (soluble) material can be performed only on the top of the blade.

[0047] Figure 9b Corresponds to Figure 6b , wherein a filling material 740 is provided; the top side of the blade is flared. Then, for example, machining of the filling (soluble) material can be performed only on the top of the blade.

[0048] Fig.9cA similar view is shown with filler material 740 provided; the top side of the blade is rectangular. Then, for example, machining of the filler (soluble) material on the top of the blade and on the sides of the blade can be completed. This will form the final profile of the top side 936c with a side gap. The shape of the side machining can depend on the interface resistance required between the forged material (first part and blade) and the cold-sprayed material. Specifically, the parameters length g, depth h of the gap, radius r, t and b of such a gap will be determined according to the strength required for the interface. For example, the length g must be wide enough and the depth h should not be too large so that the cold-sprayed material can completely fill the gap. In another preferred embodiment not shown, g can be as wide as the opposite wall, so there will be a larger gap instead of two side gaps.

[0049] The geometry of the gap is not limited to Fig.9c For example, all gap geometries that can be machined by 5-axis milling are considered. Fig.9c The gap in is represented as a rectangle, but it could also be, for example, a triangle. The geometrical definition of the gap should take into account that the machining should mainly stay in the forged material and not block the passage after the shroud is built. The only machining performed for acceptable filler material is a radius r of the same order of magnitude as that present between the blade root and the impeller hub.

[0050] In step 416, a second portion (shroud) is formed at the top surface of the filler material and at the top side of the blade, preferably by cold spraying, preferably with a material or alloy having a chemical composition that is the same or compatible with the strong bond of the wrought material.

[0051] This is shown in more detail in Fig.10 middle. Fig.10 Corresponds to Figure 8 However, a second portion 1032 is provided (which may correspond to the second portion 232, see Figure 2 ). The process of cold spraying is represented by a nozzle or spray gun 1042. As shown, the spray 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 after the filler material has dissolved provides the best surface finish for the sprayed layer.

[0052] This is also shown in the corresponding Figure 9a , Figure 9b and Fig.9c of Fig.11a , Fig.11b and Fig.11c In. Figure 9a However, in addition, a second portion 1032 is also provided (see Fig.10 ). Specifically, Fig.11cThe second portion is shown to also be present in the gap.

[0053] In optional step 418, the impeller may be heat treated. In step 420, the filler material may be removed, for example by dissolving the filler material. In optional step 422, the impeller may be heated (again). In step 424, the channel 240 (see Figure 2 ) is subjected to post-processing of the inner surface, preferably comprising at least one of the following: abrasive flow machining, hirtisation, micromachining. In this way, the surface can be further improved. In step 426, the outer geometry of the impeller can be completed by turning and milling.

Claims

1. A method for manufacturing an impeller (110, 120, 210) of a turbomachine (100), the impeller having a closed design and comprising a first portion (230, 530), a second portion (232, 1032) and blades (234, 534, 634a, 634b) arranged between the first portion (230, 530) and the second portion (232, 1032), thereby forming a channel (240) in the impeller (210), the method comprising the following steps: - providing (400) an initial stage of the impeller, comprising the first portion (230, 530) and the blades (234, 534) arranged at the first portion (230, 530); - providing (406) a filler material (740) in the space between the blades (234, 534); - forming (416) the second portion (232, 1032) at the top surface of the filler material (740) and at the top side (536, 636a, 636b, 936c) of the blade by cold spraying; and - removing (420) said filling material (740); And before providing said filling material: - machining (402) at least a portion of the inner surface of the channel to be formed to a final profile, and - pre-machining (404) the top side of the blade; Wherein, providing (406) the filling material in the space between the blades comprises the following steps: - filling (408) the space between the blades with the filling material by cold spraying, such that the filling material reaches at least to or beyond the top side of the blades; - machining (410) the top side of the blade to a final profile; and - machining (412) the filler material at at least a portion of the top surface to a desired profile of the second portion.

2. The method according to claim 1, wherein: After machining the top side of the blade, providing the filling material further comprises removing (414) remaining filling material in the top side of the blade, preferably by etching, to achieve the final profile.

3. The method according to any of the preceding claims, further comprising, before and / or after removing the filler material: heat treating (418, 422) the impeller.

4. The method according to any of the preceding claims, further comprising, after removing the filling material: post-processing (424) the inner surface of the channel, preferably comprising at least one of the following: abrasive flow machining, hirtisation, micromachining.

5. A method according to any one of the preceding claims, wherein: The filler material is a soluble material, and wherein removing the filler material includes dissolving the filler material.

6. A method according to any one of the preceding claims, wherein: The raw stage of the impeller is provided as a forged material part.

7. A method according to any one of the preceding claims, wherein: The original stage of the impeller is made of one of the following materials: aluminum, aluminum alloy, titanium, titanium alloy, nickel, nickel alloy, stainless steel.

8. A method according to any one of the preceding claims, wherein: The second portion (232, 1032) obtained by cold spraying is made of the same material as the original stage of the impeller or has a chemical composition that is strongly bonded and compatible with the material of the original stage of the impeller.

9. A method according to any one of the preceding claims, wherein: Cold spraying is performed by using a spray orientation perpendicular to the respective surface.

10. A method according to any one of the preceding claims, wherein: The top side (636a, 636b, 936c) of the blade (234, 536, 636a, 636b) has one of the following final profiles: a rectangular geometry, a trumpet geometry, a geometry with side gaps or a full gap.

11. An impeller (110, 120, 210), preferably for a turbomachine (100), the impeller having a closed design and comprising a first part, a second part and blades arranged between the first part and the second part, thereby forming a channel (240, 340, 540) in the impeller, wherein: The impeller is obtained by a method according to any one of the preceding claims.

12. A turbomachine (100) comprising an impeller (110, 120, 210) according to claim 11.

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